"fringe platforms which can't even run a Rust compiler"

> What would the Rust developers say if the LLVM project hypothetically changed its code in a way that it could no longer be used with Rust?
What if GCC tomorrow decides to drop all languages except Ada?

> Would your answer also be "Go maintain your own LLVM fork!"?
That's actually what Rust had been doing for a while. They used to maintain a private fork of LLVM with Rust-specific patches. So yes, "go and maintain your fork".

> Or if the kernel developers decided to drop support for anything but large IBM mainframes and POWER servers?
> Would you also say "No problem, I'll maintain my own kernel fork!"?
Yup.