So, this reads as, "I, the GM, get to discard your reasons (mechanical, worldview, or cultural influences) as invalid, so you cannot have it."
That is a bogus way to collaborate.
Proper negotiation would acknowledge the player reasons as valid, rather than discard them. And we'd also note the specific problems the GM has with tortles, and the ways they violate the game premises.
We then see if a way that allows a maximum of what the player wants, while engaging the fewest things the GM doesn't want, ends up palatable to both.
I don't do collaborative world building nor would I be interested in joining such a game. The players do of course change the world through their characters. If you're talking about compromise then sure we can talk but compromise requires give and take from both sides. I've been repeatedly told that anything less than allowing a tortle PC is unacceptable and for many apparently makes me a control freak.
I don't have a specific issues with tortles but I've had dozens of players in my game world over the years and if someone introduces a new species it's established as a core playable species. Given my druthers I would never play much less run a kitchen sink world because I have a hard time taking it seriously. I want species to mean something more than a rubber mask and that's already hard enough to do with a curated list. Doesn't matter to me if anyone else accepts that as a valid reason and if that means a person is so fixated on a single species and is unwilling to give at all on the concept that's fine by me.


