why fantasy kitchen sink is amazing

[Malimar]

A good DM is open to a variety of ideas, so long as they don't contradict the setting's canon or themes/motifs. If you're not willing to work with your players' ideas, go write a novel instead.

That said, it gets slightly iffy when, as my main game does, you have a large number of players cycling in and out, and each one brings ideas that are only touched on in their backstory. So I've gotta find places in the world for killorens and raptorans and goliaths and kalashtar, and then those players leave and I never do anything with the new additions to the setting because I've got my own ideas I'm trying to run with, so I've just got a bunch of old ideas clogging up the canon, mouldering, contributing nothing.

But that's mostly only a problem in a world that's small, like mine -- five tiny continents, all but one mostly explored. If your world is expansive and vast and has all sorts of unexplored nooks and crannies (the kind of setting that D&D historically has assumed), you can fit pretty much anything in without much effort, and you wouldn't expect to ever go back to touch on most ideas that come to the table.

That said, a fantasy kitchen sink is objectionable to me if it goes too far and stops being fantasy and starts being something else. To wit: I stopped being able to take Golarion at all seriously as a setting when they started in with the Wild West gunslingers and the crashed alien spaceships and ugh.