Getting 2x or 4x performance in your inner loops using a reasonable SIMD library is infinitely better than theoretically getting 8x performance with hand-coded nonportable intrinsics, because the latter is never going to happen in most programs, so the actual point of comparison is scalar code, or autovectorized code at best.

No it's not because it sucks the air out from the actual solution. ISPC more than a decade ago managed to demonstrate close-to-linear speedups for increasing vector sizes, even for branchy code.

Nowadays you can even get AI to write intristics and it works just fine, the portable libraries/autovec aren't really a serious player here.

Portability is also overstated - see the recent shift where Spotify decided to make native Android/iOS apps again instead of React Native. Usually, the number of relevant platforms is somewhere between 2 and 3, so portability concerns are more theoretical than real.