Won't this make pretty much all of the work to make a website accessible go away, as it becomes cheap enough? Why struggle to build alt content for the impaired when it can be generated just in time as needed?
Because accessibility is more than checking a box. Got a photo you took on your website? Alternative text that you wrote capturing what you see in the photo you took is accessibility done right. Alt text generated by bots is not accessibility done right unless that bot knows what you see in the photo you took and that's not likely to happen.
That’s just one part of the issue. The other part of the issue is accessibility bugs, you would have to get the model to learn to use a screen reader, and then change things as needed
And much the same for internationalization.