> I really don't understand why Intel and AMD haven't fully implemented this point: just implement the instructions that compilers use, plus the ones needed for bootstrapping and kernels. Put all the "legacy" instructions into user mode library code. It would simplify the silicon, likely reducing bugs.

They mostly have - those esoteric instructions are slower than executing the equivalent with more common instructions yourself. Its clearly the bare minimum to support back compat with the least die area possible.