I am neither a compiler writer or an OS guru. Just an old c programmer. In this article it looks like the entire CPU instruction set was designed to emulate pd

I am neither a compiler writer or an OS guru. Just an old c programmer. In this article it looks like the entire CPU instruction set was designed to emulate pdp11 to ensure c compatibility. So my naive question. What is stopping a Microprocessor manufacturer to have two instruction sets one that is compatible and one that allows us to fully utilise the modern CPU but with a different programming paradigm? Is that too expensive or hard to do? I genuinely don't know.