"microarchitecture is in large parts dictated by isa " This is very much not the case. Just consider that, for example, Intel still supports running the same I

"microarchitecture is in large parts dictated by isa " This is very much not the case. Just consider that, for example, Intel still supports running the same ISA that ran on 8086. The microarchitecture now running those instructions isn't at all like the 8086 microarchitecture. (Its running a bunch of other instructions too, but the points stands) An example in the reverse direction, is that a some MIPS chip teams saw that MIPS was dying, and reused their MIPS microarchitectural designs to build ARM processors. Sure, ISA can be designed assuming some things about microarchitecture, and then it can be inconvenient to change it. But not impossible. And during the transition to ARM64, ARM took the opportunity to remove things that were inconvenient for different microarchitectures (eg, directly accessible PC reg, changes to how condition codes work, etc)