Whereas the Far Future has a single army, divided into multiple sub armies and two major factions, serving as their flagship force as cash cow, Fantasy never quite had that same draw. You certainly had the High Elves, a force which tended to attract an inordinately large number of newcomers, but they never dominated the game to the extent the astartes did. They never had the same massive number of novels, huge wealth of fully fleshed out sub-armies and lacked the superhuman angle. By comparison, the Stormcast Eternals have just about all of that, and combined with the pauldronificiation of Fantasy's armour, it's understandably become a point of discontent.
Why are we discussing this now? Because Josh Reynolds, a longtime Black Library author and writer of many End Times and Age of Sigmar novels, had an interesting response. When the subject of their similarities was brought to him directly, he had this to say:
"Well, for starters, Space Marines are chosen as children, tortured by SCIENCE!, and then drafted into an eternity of being monastic murder machines whose sole purpose is to hold up the crumbling foundations of an omnicidal dystopia in the name of a rotting carcass that eats psykers like chiclets. They're emotionally stunted orphans who were brainwashed and weaponized before being unleashed on a galaxy where EVERYTHING is trying to kill them. They never even had a chance to be people before someone turned them into a gun instead.
Stormcast, on the other hand, are dead heroes, chosen for their valour and faith, resurrected and sent to free the Mortal Realms from the abominations currently running the show, on behalf of a benevolent god-king. They're traumatized heroes who had lives, personalities and histories prior to being crammed into primary colored hulkbuster armor and filled full of lightning so that they could go save their descendants from the eldritch horrors of a nightmare dimension. They endure death after death, losing a bit more of their soul each time, in order to prevent anyone else from suffering the fate which befell them.
One group are so far removed from humanity as to be utterly alien. The other group are so human it causes them pain. One group feels little in the way of emotion, the other group feels emotion as strongly as they did before death. One group hates and fears the alien. The other group allies regularly with space-lizards, skeletors and green monster-men. One group is the personification of the grim future in which they live. The other is a thing born of hope.
The similarities are cosmetic: big guys in easily paintable armor sell better than little dudes with fiddly bits. But the context for those cosmetic similarities is quite different. Think of it this way...Space Marines are Batman and Stormcast are Captain America. Both are super-heroes, both wear costumes, both punch bad guys, both save people. But they ain't the same, are they?"
It's an interesting answer to be sure, and it brings up the question of just where we should draw the line between inspiration and influence, and outright mimicry. While people might have cried out against this and decried their inclusion, it's hard to not cite how Warhammer 40,000 itself is an amalgamation of various direct inspirations. The astartes themselves were directly influenced by Starship Troopers, the fingerprints of Michael Moorcock can be found everywhere on its metaphysical subject matter, and even the eldar are not a wholly unique creation. However, what makes them unique is thanks to how writers and creators put a new spin upon them, distancing them from their original creations, so why are players so adamantly against the Eternals? Going purely from personal opinion, I believe it comes down to a couple of key factors.
The first among these is the shift from a gritter and more grounded fantasy setting to a a far flung cosmic tale. The death of the Old World was hard enough to handle for many, but even during that there were complaints from some that too many of Chaos' new designs seemed too 40Kified in many areas. Once the company promptly introduced a band of shoulder-pad wearing demi-god super soldiers fighting in the name of a Great Crusade to unite humanity, seeds of dissent quickly blossomed. It was, in many regards, too rapid a shift and too quick. While the End Times might have provided a clean break in the minds of some, the sudden jump to include these and extremely Khorne Berserker-esque designs was just too much. Plus, their curious resemblance to the Sanguinary Guard hardly helped matters.
Of course, even if the company had stuck to the sudden jump alone, that might have been fine, but then there's the issue of how writers present the astartes in 40,000. Despite Reynolds' statements, we have more often than not seen the astartes presented in an extremely heroic light. They're often seen as the closest the setting has to a wholly good chapter, and they're diverse enough of a faction for multiple stories to exist where they barely resemble the version cited above.
Even discounting the more moral forces, things like Rynn's World, the Ultramarines saga and a number of short stories show them to be upstanding and caring defenders of humanity rather than the Imperium. The dominance of this over the more traditional semi-psychotic super soldiers they were intended to be - and Games Workshop's desire to milk this to have more people buy them - means that there isn't enough to really differ them from one another. Plus, let's face it, we've seen them ally with certain xenos races more and more often as the years go by.
However, for the sake of argument, let's assume that the grim dark incarnation of this army is the only one which exists. Well, even then you run into more than a few distinct issues which, combined with the above example, can lead people to think that this is ultimately a Fantasy version of the space marines. Despite citing the differing origins, you can easily compare the recruiting methods of the astartes with the Stormcast Eternals. While the response might have called one conscripted children and the other venerated, ascended heroes, quite often they're depicted purely as the latter.
On many worlds, feudal or otherwise, it's an honour to be selected by the astartes, and they only choose the greatest among them to be worthy of their ranks. It's often joked, to place some real emphasis upon the astartes' overly elite nature that Conan would be one of their basic recuits, but it's not far from the truth. They would have selected someone like him in his younger days, someone powerful, bloodthirsty and skilled, to be worthy of ascension, and to them that would have been akin to being raised among the gods. The Space Wolves, Mortifators and many others have traditions which directly resemble this, and once you pick out that point the similarities start to become more and more distinct. Replace Sigmar with a primarch/Emperor and his power with gene-seed/conditioning, and you end up with a vast number of parallels between the two forces.
Now, despite the similarities, I personally don't think that's the greatest issue which is causing so many problems here. You can have many very similar forces, very similar structured building blocks and still end up with something completely different. As they're structured, as they're presented to a degree in their lore, the Stormcast Eternals are the more angelic choice here. They're figures who once you really look at the nuances and elements building up each army, you see how they have started at the same key point but quickly split off from one another. Their very link to the Old World and alliances alone are enough to break them up from one another once you really examine it, but on the surface it might still seem extremely similar.
It's the same problem Pacific Rim suffered from once it was leapt upon by shrieking bloody fanatics of Neon Genesis Evangelion for being an "Americanized rip-off." Because Del Toro used certain setting elements and piloting systems which retained some initial comparisons with the anime, in the minds of many that instantly damned it. They demeaned and insulted any who enjoyed it as supporting plagarised content, without bothering to look into how each universe branched off from one another. However, such surface scans of a material, no matter how many details you might be covering, can always result in very different shows seeming identical at first glance.
Don't believe this? Well then, read the following:
You have a world which is perpetually under threat from an enemy alien race. A race which can emerge instantly on terrestrial earth with no need for space travel and retains religious overtones purely for the sake of style. Their weapons of war and very bodies seem monsterous, like creatures of legend or beings warped beyond the point of ever resembling humankind.
All that stands in the way of this threat is an organised military force operating out of a heavily defended city, built to instantly slide into underground bunkers the moment it is attacked. While retaining many fortified military units, their one hope for true victory in this war relies super-prototype war machines equal and opposite to the aliens, which are fired at high speed into battle from their base. Among their crew is a unique figure with a direct genetic link to their foe, and the offspring of the organisation's often difficult commander.
Now, am I talking about Evangelion or am I talking about Stingray? Could easily be either despite how that detailed description seems to uniquely fit only one series on first glance.
As a result of this, I personally feel that the fault lies more with the execution and presentation of the Stormcast Eternals than anything else. They're certainly inspired by the astartes to be sure, but there's enough of a separation to make them stand on their own. However, that's not being depicted at this point. Currently, Games Workshop is focusing upon selling the Stormcast Eternals purely upon their image, their basic look and little else. There's no nuance present there and no opportunity to really examine their culture for lack of a better expression. This sadly carries over to more than a few stories, as while Mr Reynolds might have a firm grip on what makes them tick, other authors seem to slide into the thought processes behind writing astartes. As such, it can seem to fans like they're reading about a chapter but sans the traditions, solid identity and ideologies which helped make them so distinct.
To really avoid the continued labels which have plagued this new army, Games Workshop really needs to help present them in a broader sense. They need something compatible with the Index Astartes, something easily accessible, well written and in depth, delving into enough of their history to make them seem truly unique. They need more opportunities to be shown on an individual basis, covering the various hosts one by one rather than just sticking to the gold clad Hammers of Sigmar. If they were truly able to present this to the fandom as a whole, it seems far less likely we'd be seeing so many accusations of one army being a transplanted version of another.