Noone uses an 8051 because it's elegant. Billions are still still sold every year because no matter if you learned it in the 70s or last week and no matter who made it, the basics are exactly alike. Software matters; ISAs don't.

I believe the market will standardize on certain extensions for specific solutions. No one is going to make a mobile phone with only RV32I, for example.

How can you use consistency and SSE/AVX in the same sentence?

SSE has inconsistencies like SSE4.x vs SSE4a. AVX is an even more mixed bag. There are some 19 AVX-512 extensions and ZERO chips support all of them.

The situation is so bad that AMD and Intel got together to make AVX10 to unify everything. That seemed great, but Intel now has AVX 10.1 and 10.2 in addition to the base set, so there we go again...

x86 is a massive battleground with tons of competing extensions like FMA3 vs FMA4 (why did FMA3 win???) and in cases where one of the competing variants didn't win, we get something like virtualization extensions being completely different between Intel and AMD. There's also the rash of security extensions that have gone through various support and dropped support (not to mention using some of this stuff for market segmentation and further fragmenting the ecosystem).

x86 is anything but consistent if you look into its history (or even it's present).

> Because nobody will write software for 300 unique hardware variations

Who said they have to? One can select a RISC-V configuration for a baseline for a particular purpose. Desktop? Choose the one that's most powerful.

ARM is more popular than x86 and is less consistent than it.