[roca]

It's not just about legacy instructions which can be decoded down to some microcode. It's about architectural features like SGX, CET, MPX, TSX, VT --- plus the legacy stuff like segment registers and 286 call gates and virtual 8086 mode and so on and so on --- and how they all interact with each other, and how they increase the complexity of context switching, OS support, and so on.

[kllrnohj]

OS complexity is because they choose to keep and value that backwards compatibility, just like x86 values backwards compatibility.

Linux could eliminate all that complexity tomorrow by just bulk removing x86 support and only running x86-64. All OS complexity eliminated. All context switching complexity eliminated.