There is no natural life cycle for an ISA. (I am looking at you, x86!) It is easiest to pretend there is such a thing, because we are socially attuned for that

There is no natural life cycle for an ISA. (I am looking at you, x86!) It is easiest to pretend there is such a thing, because we are socially attuned for that. But it is not a technical observation, but a sociological one. And, that was my point. We have maybe learned a few things about ISA design since POWER was cast. But we have also forgotten things. It is far from clear we are ahead. Most of what we learned feeds into stuff like vector units that have little to do with whatever they are bolted onto. One example is the popcount instruction, omitted from the base RISC-V, and from all chips when last I checked. It has been added to every architecture ever mainstreamed, always at great expense, but it seems hard for ISA designers to contemplate. There is a special extension just for it so it could be put into a standard "profile" without importing the entire B thing.