So far the situation with ActivityPub is that the protocol is flexible enough to allow very different feature sets and user experiences. The most popular so far are twitter-like and reddit-like, with multiple implementations of each. I don't think ActivityPub was designed with the reddit-like use case in mind, yet it works well for that. There's no user revolt because the creation of new software with new experiences doesn't have much impact on users of existing software.
Enshittification is hindered not because nobody could create a Mastodon fork (or green-field project speaking the same protocol) that's riddled with ads, but because people can select a different service provider and still access the same network.
Enshittification is hindered not because nobody could create a Mastodon fork (or green-field project speaking the same protocol) that's riddled with ads, but because people can select a different service provider and still access the same network.