If the websites bother to export RSS/ATOM/ActivityPub feeds, they won't need to. They'll just "subscribe" to the stream in their preferred app/webservice, and get aggregated updates about everyone they care about.
Sure, but what about commenting on the post? No one likes creating accounts for different websites in order to post one comment.
It's obvious that things like Twitter and Instagram provide value to celebrities and people who follow them. It's just that there are some serious externalities not factored in.
ActivityPub lets you mark a comment as a "reply" to a post (or any web resource), then all you need is a trackback-like facility to link from the post to comments on it. It doesn't need to be centralized.
