People end up leaving or not participating subreddits with low-quality moderation. They also tend to get overrun with spam and abuse, and have to be shut down. Finding a bunch of crappy replacement moderators isn't going to work long-term.
The other part of my prediction is that some of the (real, non-bot) engagement that the giant generic subreddits get are from users that are anchored to Reddit by more specialized subreddits, like those for specific games, hobbies, that sort of thing. If those special interest subreddits die out, I think the large generic subreddits will become more hollowed out than they are already, and not worth much.