Thank you for your insight. This is an interesting take to me.
I have regularly gently reminded multiple people to add alt text to their posts. One of them got pretty upset about the whole thing, and I ended up unfollowing them because I wasn't comfortable with their antagonistic attitude towards accessibility.
In every other case beside that one, when I've reminded people, they've all responded positively and began regularly incorporating alt text into their posts.
But I suppose that simply providing a description in a reply with an appropriate hashtag might be better, as it would also serve as perhaps an even more gentle reminder.