There is a huge business reason to restrict the isa. simple stable interfaces are very attractive to users(buyers). Take amd64 as a counter example, not only d

There is a huge business reason to restrict the isa. simple stable interfaces are very attractive to users(buyers). Take amd64 as a counter example, not only does it not take 4 million lines of code to interface with an amd64 processor, when something new is added the processor is still compatible with existing code. x86 as an architecture is a bit of a train wreak but much of it's success is the effort to keep the interface stable. Also for example arm, this is getting better but the hardest part about using arm is the isa is all over the place, every single bloody SBC appears to have a different isa.