This is a fair point. However, I actually think GCC's negativity and steadfast refusal to adapt and evolve contributed more to its rise than anything else.

Apple may have brought users. But the contributors are mostly ex-gcc folks, or folks from research communities/other compiler people. They are the people that were/would be working on GCC if LLVM didn't exist.

The reason LLVM got the majority of these contributors is not because Apple made it the default compiler, or even because Apple supported it.

It's because GCC was unwilling to change and adapt in the ways that its contributors (current and future) wanted. For example, becoming a modular, pluggable compiler. RMS resisted plugins for many many years. He resisted writing out the IR. He resisted requiring newer versions of C, or C++, or ... (While GCC is led by a steering committee, RMS can override them, and the recourse is to go to the FSF board or something. I can't remember all the details of the EGCS reunification agreement). The steering committee of GCC, while all wonderful people, over time became less of GCC contributors, but refused to turn over their steering committee roles to any younger and newer contributors. This left GCC with a steering committee that, while well intentioned, and again good people, was not in any way related to the current major contributors. Part of this was just politics. GCC simply had a lot of politics (because of the FSF, because of EGCS, etc) that LLVM did not.

In any case, contributors and researchers left GCC because of this unwillingness to change. This is what caused a community to coalesce around LLVM, not Apple. People in this community advocated to their companies to adopt LLVM, and some were successful. This is why Apple uses it. This is why ARM uses it. It is because the community and it's members were successful in convincing them to do so (certainly, there is more, in that the companies saw long term strategic benefit, etc)