A small number of mods moderate the overwhelming amount of subreddits and posts on the platform (some moderate hundreds of subs), and those are doing just fine. At least for the most popular subreddits, nothing will materially change.
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.