You know, when I homebrew a world for a campaign, the ancestries I allow aren't just a list of things I think are cool. I design the world around my whitelists and blacklists. They have cultures, and territories, and allies and enemies. If a common element is absent, I've planned around its absence.
So yeah, if I know a player wants something... I'm going to include it from the jump because I have to; if someone wants to play something I haven't accounted for, I have to do a lot of reworking to fit it in.
One of the reasons I do whitelist/blacklist, either on my own or as part of collaborative gameplay, is that the D&D rules have way too many ancestries for the majority of D&D settings, and even in Spelljammer and Planescape there's a limit to how many obscure or niche ancestries can be supported in a game without losing... a sort of cohesion in how they all fit together. What I allow people to play as PCs has to be reflected in the prejudices and behavior of NPCs and if more than one (or two, in a big group) of the PCs is Outside Context, that part of the game-- that facet of immersion-- becomes unsustainable for me.
I'm more flexible than most, but I have limits. And I don't think it's a coincidence that the people who object to mechanical and narrative restrictions on PC ancestries, and limitations on allowable PC ancestries... are largely the same people that argue that "flavor text isn't rules" and that the rules should take precedence over what the rules represent in the fiction.
Which is precisely what I like least about the evolution of the Modern D&D and D&D adjacent game design and play cultures over the past 25 years.
