This cuts both ways: a vendor willing to fix things can if it is just about updating a library, but then a vendor that doesn't give a crap and who will never roll out fixes leaves a user stranded. But if the fix is part of the OS, every user will benefit from it, regardless of his device manufacturer.

I think in practise we have a mix anyway - manufacturers already license BT stacks from a few vendors, and don't develop them themselves.