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.
Where is a good feed finder?
To look forwards, you must look backwards, at new forum software such as NodeBB or Flarum. Specialist niche communities are already shifting in that direction, or have never left their old BBSes behind, but are due to welcome a resurgence of new members.
When a friend referred me to hackernews, I found that it has the type of content and users that I miss from reddit. I also visit Tildes on occasion, but the user base is so small that the content is much more limited than it is here.