Yes, I'd say that the strongest part of autovectorization is that you can get more-or-less automatic support for wider/newer instruction sets than what you had

Yes, I'd say that the strongest part of autovectorization is that you can get more-or-less automatic support for wider/newer instruction sets than what you had when you wrote the code; older SIMD, like all other code, tends to rot. Of course, this is predicated on either having function multiversioning (works, but very rare) or being able to frequently raise the minimum target for your binary.