I don’t think they’re likely to do a better job than humans any time soon.

Sure, assuming the human is actually putting effort into the task. But we know that able-bodied society is generally, at best, dismissive of the needs of the disabled and, at worst, discriminatory. I very much doubt that the majority of fully sighted humans working in this area are taking the time required to view the problem from the point of view of the visually-impaired minority and then putting in the effort required to deliver the best possible solution for them. Not every website is run by some massive company with employees specifically dedicated to this task. For many it will be an afterthought, and that's where AI descriptions will shit all over the lazy human ones. Additionally, alt text contributes to SEO which means many will be tailoring it to their search ranking instead of the needs of the user.