I won't make the argument against variable-length vectors here, but I will note that, although some of the extensions you mention do change the vector length, f

I won't make the argument against variable-length vectors here, but I will note that, although some of the extensions you mention do change the vector length, for none of them is the vector length change the most interesting aspect; you could keep the vector length fixed, and they would all still be interesting. (The exception is avx10, but that is basically forwards-compatible and barely an 'extension', so.) Contrariwise, although I think some of amd's choices in the initial design of amd64 have hurt its longevity—they could have made more breaks from 32-bit x86, although it is understandable why they didn't at the time—intel has been very careful and has done an extremely good job of playing the hand it's been dealt.