There are, apparently, 2.8 million subreddits. About 8,000 are dark, meaning that's just over one quarter of one percent of subreddits. Even with some of the largest subs participating, I wouldn't be surprised if there is no massive dip in traffic. There are enough subs still open with enough mass appeal that most people will just look at some other subs for a few days. And I'll be honest, even though I've made an account both here and on kbin, I know I'll still use old.reddit (with RES and an adblocker at least) most of the time, simply because I doubt any of the subs I actively look at will do any meaningful migration that would lead to a similar level of discussion.