Most fantasy to the 1990s was sub-Tolkein. DnD is very obviously Tolkeinesque - sure they borrowed stuff from elsewhere too, and made up some extra creatures, but no-one can really pretend they were heaeding anywhere out of the comfort zone (e.g. just the basics of diminished elder races of elves in forests and dwarfs in mountains and orcs / trolls / etc. roaming around messing shit up plus quests and grand battles). Sure, some of it has a different tone, but the fundamentals are that fantasy was pretty much re-doing the Tolkein basics over and over for decades. WFB is incredibly obviously smack bang in the the most routine of sub-Tolkein cliches. Chaos of course essentially take the place of the more conventional "Dark Lord" threatening to dominate the world.
I agree, but simultaniously don't.
I don't have the time or inclination to go through every similarity and difference between LOTR and WFB, but I'll try and be as succinct as possible.
On one hand, yes, there's obvious Tolkien influences in the setting, via the stock fantasy races (elves, dwarfs, orcs, goblins, ogres, halfings, etc.) Whatever differences may exist between them, the tropes are still there.
On the other hand, there's stuff in WFB that owes nothing to Tolkien - Lizardmen, skaven, Vampire Counts, Tomb Kings, not to mention that while both settings use human cultures as inspiration, there isn't that much overlap between the sources of said inspiration (Tolkien wouldn't be caught dead using Arthurian inspiration ala Bretonnians for instance). A lot of these are tropes and/or real-world inspirations (though skaven seem to be pretty unique), but whatever their sources, LOTR isn't among them.
On the third hand, I also think it's a stretch to compare Chaos to the Dark Lord trope. There's some similarities, sure, and from the outsider's perspective you could argue that the differences aren't without distinction (rule/destroy the world), but even then, I don't think it's 1:1 in that:
1: Tolkien's dark lords (Morgoth, Sauron) are primarily Abrahamic in their inspiration. Morgoth is basically Lucifer (angel/maia that rebels against God/Illuvatar, sullies Earth/Arda, will return on Judgement Day), and Sauron is basically Morgoth-lite. Evil in LotR often runs with the 'fallen' motif, of good being corrupted (the orcs are a case in point).
2: To me, the Chaos gods are more akin to Lovecraft, or that trope of "unknowable entities with unknowable goals." There's nothing 'fallen' about the Chaos gods, and there's no particular Abrahamic inspiration (yes, you could compare the Realm of Chaos to Hell, but onyl in the broadest sense). And while both Chaos and Morgoth corrupt, the handling of them is different - Chaos corrupts on a literal level that can change flora, fauna, even the landscape itself. Morgoth/Sauron corrupt more on the spiritual level.
By extension of this, I'd say there's another key distinction in that LotR is largely a moral binary world, where WFB is very much shades of grey. There's also the thematic differences, but that's a whole other kettle of fish.
The TL, DR version is that yes, obviously WFB is inspired by Tolkien in a number of ways, but there's plenty of stuff it doesn't owe to Tolkien. And while we may disagree, I'd call it one of the most fleshed out worlds in fantasy fiction, regardless as to how original that world might be.
Space opera is epic fantasy in space. Or epic fantasy is space opera on a planet with magic. 40k is WFB in space, right up to the point all the stock fantasy archetypes were translated straight across. Elves (eldar), orcs (orks), hobbits (ratlings), dwarfs (squats), ogres (ogryn). There's even a shared quasi-pre-history of intelligent lizards (Slann). WFB pretty much could literally be just one lost planet in the 40k galaxy except the races there lost their tech and declined to primitivism. It has of course diverged a little, but hardly radically and without much conspicuous imaginative novelty. Of the "new" races in 40k, they just copied across the undead (Necrons), Aliens from the movies (Tyranids/genestealers), and the dwarfs again (killing off the originals and resurrecting the concept as the Demiurg - although I almost cannot fucking believe this but apparently GW have just retconned the Demiurg back into being squats.)
Really don't see the necrons as undead, and you left out the tau, but that isn't the crux of your argument, so I'll move on.
This is why I don't really have any time for some weird insistence that it would be unacceptable to make female space marines because it would offend against the lore. The lore is such junk, (badly) written then re-written and retconned and confused and compromised, that it just doesn't merit that much respect.
Okay, here's my crux:
1: I don't really buy the argument of "x is bad, so don't care about x." If the lore of a setting is haphazard, that's cause for being more congruent with the setting, not just throwing up your hands and giving in.
2: I've already given my issues with female Space Marines many times from the perspective of the universe itself, and to be clear, if female Space Marines ended up in the setting tomorrow, there'd be far more egregious shifts in my eyes (e.g. the necron retcon thing, the Primaris marines are a similar asspull, etc.) But the difference here is the ideological underpinnings behind a lot of it. You may roll your eyes, but here's the facts:
a) The call for female Space Marines often comes from outside the fanbase itself.
b) Calls for female Space Marines are never accompanied by calls for male Sisters of Battle (we can extend this to other all male/female orders in the setting, but these are the big two)
c) If there's a reason why there's a "need" for female Space Marines, but other single-gendered orders are fine, the people demanding the change owe an explanation. Is it something about the Space Marines by themselves that demands this change, or are we operating under the principle of single male orders are a no-no, single female orders are fine?
The assertion that Warhammer is KFC is an assertion I actually don't have too much of a problem with, but let's extend this to some more 'prestiguous' IPs. I can cite all-female orders in fiction (Bene Jesserit, Aes Sedai), as well as all male ones (Night's Watch). I've never encountered anyone demanding that these facts change. And even if you make the argument that 40K is less "prestiguous" than those IPs, well, okay then, we can look at all female groups in IPs of similar standing (MLP, Sailor Moon, WITCH, etc.) Do they need to change?