> it still looks like the evolution of one ISA to me. There is a much greater degree
> of backward compatibility here then say differing Power families. But I am sure
> you will contest that. ;)

Absolutely ;-) PPC64 and PPC32 are much more alike than x86-64 and x86(-32) or AArch64 and AArch32. The reason for the good backward compatibility in the latter cases is the fact that these 64-bit processors separately implement the 32-bit ISA in addition.