What does open source have to do with this?

That’s a really long stretch there.

Who, when creating new OSes focuses on really old, consumer, nothing unusual hardware?

Second, for compilers you have projects like LLVM that's like a framework for building compilers where those companies like Intel, AMD, Samsung, etc contribute.

Not everyone has the opportunity to only have new computers, especially in many third world countries.

LLVM shouldn't turn into compiler monoculture.