> Many people already use non-portable SIMD for the 1-3 targets they care about, instead.

This is something a lot of people (myself included) have gotten tripped up by. Non-portable SIMD intrinsics have been stable under std::arch for a long time. Obviously they aren't nearly as nice to hold, but if you're in a place where you need explicit SIMD speed-ups, that probably isn't a killer.

Exactly. Many parts of SIMD are entirely stable, for x86, ARM, WebAssembly...

The thing that isn't stable in the standard library is the portable abstraction layer atop those. But several of those exist in the community.