>You learned it in the 70s [emphasis mine]
I really don't know anything about this space, but you just said that 8051 is dominant because it has one dominant architecture since the 70s. It has hundreds of manufacturers making identical parts.
As you say, software matters. If the Software can't run because of hundreds of extensions that can't be checked for, then you're going to pick a target that works, no? So in fact the ISA matters most: which ISA has the most software? Which ISA means my software runs on the most devices?
I really don't know anything about this space
And yet you couldn't help yourself...
but you just said that 8051 is dominant
I said absolutely no such thing.
If the Software can't run because of hundreds of extensions
The software in the x86 world runs because there aren't hundreds of mutually incompatible extensions. I think the last time there was a major completely incompatible x86 ISA divergence was AMD "3DNow" vs other SIMD extensions. AFAIK the rest were "processor X got feature Y later than competitor Z".
Which ISA means my software runs on the most devices?
Easiest question evah: x86.