Mercurius
Legend
I have two answer to this.For all this debate about cosmology and multiverses I have to ask - does it actually come up in the game?
Is your Bard in Waterdeep concerned with the First World?
When you're Cleric visits the Inn of the Last Home does the ecological needs of a sapphire dragon in Eberron matter?
1. For the most part, no. Or rather, as a DM, my observation is that it is secondary to hanging out, rolling dice, and adventuring--and some players don't care about such things at all. But it helps to bring color to game, and I find that as a player, I'm very interested in such things, and always want to know more about the world. In fact, I tend towards disappointment if I feel that the world is paper-thin, be it the depth of the DM's homebrew world-building or their understanding of the pre-published world. But even then, I am generally happy to play casually, and as a game of fun and adventure, with any kind of world elements being back-drops only.
2. I'll tell you a secret - and don't tell anyone else on this forum, else I be cast to the nether regions for heresy. Playing RPGs is actually a tertiary hobby for me; or rather, it is a primary hobby, but two related areas take primacy in my life as art forms: Writing stories and world-building (for the stories). My interest in RPGs is partially due to that; it is also why I tend to buy settings books over other books, because I find more enjoyment from them outside of the game context than other books.
I partially jest about the tertiary thing, but I think it illustrates the nature of RPGs: They are different things to different people. There is a certain attitude that sometimes arises, that almost looks down on anything that doesn't immediately impact the game play. Actually, this also exists in the writing world; M John Harrison somewhat famously called world-building "the great clomping foot of nerdism." He was coming from a place of "pure storytelling," so--as far as I understand his meaning (aside from the humor of it)--he was critiquing excessive world-building that has nothing to do with the unfolding of the plot.
While I think he had a good point, his view is not only a bit snobbish but also misunderstands someone like, I don't know, JRR Tolkien. For Tolkien, his main work was Middle-earth itself - it was an ongoing art project, like an extremely complex imaginal sculpture that he continued tinkered with and refined. The books he wrote were rather secondary to the world itself.
I think some DMs have a degree of this "Tolkienism," and it is best understood as a related hobby--or art-form--that overlaps with RPGs, but isn't the same thing. And I think the same applies to your comment about cosmologies and such (not saying you are being snobbish, btw!).