My (inexpert) view is that it might be the last ISA because a general purpose ISA will become less important. I see the cpu core as being more like a glue that connects together specialist hardware, so you have a wide SIMD number cruncher (like AVX) but probably off-core, likewise off-core compression (perhaps), encryption, graphics (already there really), prob matrix mult, all the fancy hardware for AI that's coming out now, possibly embedded prolog/logical deduction device etc.

So the main ISA will be little more than a distribution device for allocating tasks to subunits, and no-one will care how it's put together.

But I'm no expert.