Fair, and there are also of course ISAs that could be drastically impactful like the ill-fated Itanium or the doesn't-seem-to-be-real Mill.

Agreed. I think it's hard so see a new ISA getting much traction now unless it clearly offers a very significant power / performance benefit and that seems unlikely at the moment. I blogged that RISC-V might be the last mainstream ISA a while ago - I think it will crowd out other options.