@skye Several blind folks on Tw*tter explained it's rather impossible for them to find image descriptions hidden in replies. It may work if OP replies to their own tweet, and thus creating a thread. But it doesn't work well with random replies from other people.
@katzentratschen that's true -- and its even worse with federation (where they might not see the reply). That said I've also seen blind folks (both here and Twitter) who are for some reason unable to caption pictures point out just how hostile an environment it is when they get told to do so by a sighted person.
@skye another advantage of your approach: it provides text for the OP to cut-and-paste and add to alt text, which makes it a lot easier for them.