Itanium also did it by slapping on another die that was an x86 cpu. The core Itanium instruction set where totally new and different at the time. If the new th

Itanium also did it by slapping on another die that was an x86 cpu. The core Itanium instruction set where totally new and different at the time. If the new thing were and x86_64 CPU with just the oldest, least used and least useful features removed, things that haven't been used intentionally by new software in the past 15 years the problems you describe could largely be avoided. The gains would not be as big as cutting out the all worst features, but there would be some gains for minimal pain. Potentially this could be paired with a deprecation process that Intel, AMD and other x86 (Are there any other) manufacturers agree on in an open setting with ISVs. If this working group announces the deprecation of some features and the ISVs agree its a good idea, then the chip makers could put out a date some years in the future where that feature would no longer be in new CPUs.