Apple is not the largest contributor to LLVM, Google is. The various other non-Apple contributors also outweigh Apple. It would be hugely expensive for Apple to hire all those compiler developers from Google and Intel and dozens of other companies. Why would they do it? What would it gain them other than to be comically villainous.
The reality is that GCC is stagnating because a lot of things people want to do are just not very easy to do with GCC, due to architectural decisions that were made for purely political reasons (and these arguments have been going on since long before LLVM even existed).
It used to be that academics, students, post grads and hackers who wanted to do something interesting and new with a compiler would start with GCC. That hasn't been true for some years now. Pretty much all the interesting new projects are based on LLVM. And that isn't because of Apple paying people. It's because LLVM has modular design that gives you a stable clean interface to various things while GCC only offers unstable and complicated internal data structures.
So increasingly people interested in studying and developing compilers are hacking on LLVM and GCC is left only with developers who have a very strong political agenda that motivates them to contribute to GCC. Unfortunately for RMS most compiler developers don't work for free out of their basements. They are well paid and work at large companies and so any emotional or political arguments about freedom don't get any weight with their bosses compared with the ability to actually get shit done. So they are switching to working on LLVM.
As detailed elsewhere in these comments RMS knows full well that the political decisions limit the functionality of GCC. He says he is happy to pay that price to prevent anybody using GCC in a project that isn't GPL licensed.