If problems "a" and "b" above have been addressed, this doesn't matter. Developers can simply delegate part of the decision process to some UI expert they already know and trust.
Compare
"Your UI improvement sounds interesting but it's too risky to trust a random UI expert. Sorry."
To
"I ran the idea by X and they mentioned that accessibility would become a problem with your approach. How do you plan to address that?"
IMO one sounds more likely to attract quality contributions than the other.
One additional problem is that it's hard to know who to listen to. Someone who seems like a confident UI expert might as well be a misguided visionary who will turn GIMP into the next Windows 8. Randomly trusting a designer is probably just as dangerous as randomly trusting a software developer.