For example, I preferred some of the more tech-related subreddits. Then I discovered HackerNews and Lobste.rs. That can't be the only demographic of content that has forums out there. And I imagine more will pop up.
Again, it's just my prediction/guess. I've certainly been wrong before. Perhaps I have it totally backwards. Maybe those people will stay because they're more tolerant of Reddit's bullshit to get to the good content. I don't think that's a smart bet for Reddit to make, though.
EDIT: Also, a lot of people are apparently moving to Discord chats.