> It's not totally clear to me why simt won out over writing the vector operations.

From the user side, it is probably simpler to write an algorithm once without vectors, and have a compiler translate it to every vector ISA it supports, rather than to deal with each ISA by hand.

Besides, in many situations, having the algorithm executed sequentially or in parallel is irrelevant to the algorithm itself, so why introduce that concern?

> I'm certainly in the minority opinion here.

There are definitely more userland programmers than compiler/numerical library ones.