Of course it's not 1:1, otherwise it would receive a copyright suit. It is necessary to change something in order to create something new, but the fundamental shell of WFB is Tolkeinesque. Much like if you redecorate a house it might look different in lots of ways, but it's still got the same structure.
I think it's highly debatable on the idea of the "fundamental shell" being the same, but that's not the crux of the argument. So on that note:
A dark lord is a fantasy trope of an evil (usually divine) power that poses an existential threat to the world, intent on utter subjugation or destruction. (This might be in comparison to a sort of "realpolitik" view of the world). How is this not Chaos? Plus that Chaos in the Warhammer world is unambiguously evil in nature: malevolence, betrayal, greed, selfishness, torment, corruption, killing, etc.
-Re-read what I wrote above as to why I distinguish between the two. You've pointed out in your own post that Chaos is "fundamentally evil," which is not a charge that can be levied against Morgoth or Sauron.
You're right in that the end goals are arguably the same - LotR 'lords' want to rule the world, Chaos wants to destroy it. Fair enough, if that's all it takes to be a dark lord, then sure, they fit the bill. But to be as succinct as possible, both in-universe and out-universe:
1: LotR lords take inspiration from Abrahamic mythology, Chaos is more eldritch in nature (I'd say Lovecraftian, but I don't think they quite fit the bill either.
2: LotR lords are fallen individuals (more on that later), Chaos is a fundamental force of nature within the setting.
3: The nature of corruption in LotR is more on a spiritual level, or when it manifests physically (e.g. orcs), it's done with direct intent. In contrast, Chaos corruption is mostly on a physical, undirected level
Basically, I call Morgoth/Sauron/Voldemort/Brona/Shadow Lord/whatever "dark lords" in that they're distinct individuals with distinct goals, with distinct human failings. I don't call Chaos "dark lords" for the same reason I wouldn't call C'thulu a dark lord, in that we're dealing with forces of nature that we can't comprehend, that are above comprehension, that can't be negotiated with or whatnot. In essence, it's the individual vs. the element.[/quote]
This is unsupportable. We can, for instance, hardly fail to draw parallels between Aragorn and King Arthur, and Gandalf and Merlin. Tolkein was interested in early British myth; he was involved in scholarly work on Arthurian legend, and wrote a very long poem on King Arthur
(unfinished). Tolkein may not have viewed Arthurian myth as a suitable main basis for the world-building he envisaged, but it seems unuspportable to think he did not incorporate elements of it.
To quote Tolkien:
"I was from early days grieved by the poverty of my own beloved country: it had no stories of its own, not of the quality that I sought, and found in legends of other lands. There was Greek, and Celtic, and Romance, Germanic, Scandinavian, and Finnish; but nothing English, save impoverished chap-book stuff. Of course there was and is all the Arthurian world, but powerful as it is, it is not perfectly naturalized, associated with the soil of Britain but not with English; and does not replace what I felt to be missing. For one thing its 'faerie' is too lavish, and fantastical, incoherent and repetitive. For another and more important thing: it is involved in, and explicitly contains the Christian religion. For reasons which I will not elaborate, that seems to me fatal."
It's generally accepted that the purpose of Lord of the Rings was to create an "English mythology" - as in, something pre-Arthurian, pre-Norman, going back to the Celts (not sure where the Anglo Saxons fit in), etc. Obviously Tolkien was aware of King Arthur, but LotR itself is intended to be its own thing, even if it still borrows from other cultures/myths (Christian, Norse, etc.)
As for Aragorn and Gandalf matching Arthur/Merlin, again, I disagree. Sure, there's similarities, but that's like saying LotR and Narnia are similar because they both have dwarfs in them. Similar tropes, executed far differently, and with different themes behind them.
I'm not entirely sure where your knowledge of Warhammer lore starts, but for me it was the mid-late 80s. Bretonnia was not principally modelled on Arthurian myth: that was a later development over the ~40 years they had to flesh out their creation (again, by particularly unadventurous borrowing). The Old World is just a take-off of medieval Europe, with Bretonnia as a sort of Hundred Years War Eng/Fra mash-up. Then Estalia as Spain, The Empire as the HRE, Tilea as Italy, Kislev as (Kievan?) Rus, etc. The names kind of give it away.
I was first introduced to Warhammer in the 90s, but it's academic as to when the Arthurian stuff came in - lots of fictional settings have lore added to them over time. Heck, Lord of the Rings is a case in point.
As for the nations of the setting taking inspiration from real-world ones...yes, and? Of course, that was part of my point, the inspirations for WFB nations don't have that much overlap with LotR ones. Numerous fantasy settings take inspiration from real-world cultures, I don't see that as a mark against them.
Again, as above.
GW have had 40 years to develop their product. But I remember the 80s, before a lot of that existed. The basic history is elder races of elves and dwarfs, they have a big war, diminish in the face of the rise of humanity and orc incursions. Elves mostly live in forests, dwarfs in mountains, except that island/continent with elves off overseas to the West (!). Even then, there are three types of elves:
Noldor High elves, Sindar Wood elves, Teleri Sea elves. Plus ents, "halflings", etc. All these races are of course straight out of Tolkein cliche. Then just mix it up with a different flavour of dark lord(s), model human realms as pastiches of a few medieval European countries and add a couple other races. Lore done.
Except the product HAS been developed over the last 40 years. You yourself point out that the Bretonnia of the 80s isn't the same as the Bretonnia of the 90s/2000s. You're also right in pointing out that stuff like elves, dwarves, orcs, etc., are Tolkien-esque tropes, but you've completely sidestepped the factions that have nothing to do with Tolkien.
There's also another point to make - you're conflating "effort" with "originality." I'm quite happy in saying that something like Bretonnia isn't that original (Arthurian legend meshed with France), that isn't the same thing as there being no effort involved. I mean, you CAN make that claim, but I'm not sure how one would come to that conclusion after reading an army book or browsing the wiki. By the logic you're presenting, something like A Song of Ice and Fire would be "low effort" since it's borrowing from the same sources, or something like Ranger's Apprentice would be on the same scale as WFB since they both draw on Arthurian legend, despite there being vast differences in the depth of the lore.