The Linux community is way way way larger than just the sum of distro user bases.
Even if all distros have long ago moved to GCC 8+, you still have other chips and systems running, and who knows what buried somewhere depends on GCC 5.
You'll have to upgrade the compiler eventually though, right? I don't know what that process would look like, but generally the longer you wait the more pain you'll face later.
Compiler tech and hardware will improve drastically in the next 100 years. Linux can either evolve with it, or be put in a museum while something else takes it's place. Likely losing a lot of the development that's been done in the past 30 years.
It would be like running OS/8 (for the PDP-8, popular in 1965) on a modern machine. No time sharing for processes, no network stack, only a TTY for I/O... what would you even do with it if you could compile it?
We’re clearly talking about very different things.
If someone made a product which uses it, and people keep buying it, most companies aren’t going to bother with any of those things if they don’t need it.
That is of course completely different from what Linux is doing (as in outside of that context). It may be at version 510, but that doesn’t mean someone won’t be running version 1.0 somewhere. And that’s fine.
Even if all distros have long ago moved to GCC 8+, you still have other chips and systems running, and who knows what buried somewhere depends on GCC 5.