That is irrelevant, as most of those companies are keeping their modifications to LLVM closed-source. They are not at all interested in fostering a healthy open-source ecosystem around LLVM.
> GCC doesn't really have that.
This is just wrong. Just as a single example, the first ever C++ concepts implementation was written for GCC by Andrew Sutton, a University professor and co-author with Bjarne of the concepts proposal. LLVM's implementation came years later, and it's not as stable or complete to this day.
> If Red-Hat and other private companies withdraw support, there are really few people familiar enough with GCC to keep it afloat, few people interested in learning and contributing because that won't land them industry jobs, etc.
Again, no. There is nothing in the 30+-years long history of GCC that suggests that this would happen.