> The biggest problem is that it would be a breaking change at least in C++.

Exactly, I believe that to be the case in C as well. In C23 the rules for types to be considered compatible were actually relaxed, so this proposal wouldn't make any sense. It would be a useless breaking change for no gain, other than a bit less typing and maybe feeding some language layers, so there is really no reason to do that. It actually makes the code less clear, since you now need to always lookup the type definition, which would be against C's philosophy.