Only some of the subreddits are going to stay dark unfortunately. The mods are worried that if they stay private too soon, scab redditors will make their own subreddits and take their niche's users.
Probably for the best. So many reddit users are overly pejorative to moderators because of bad experiences. Let the current moderators get replaced and maybe people will appreciate the previous experience.
Maybe then, reddit will get off its ass and build proper tooling for moderators.
Once third-party tools are gone those huge subreddits will degrade quickly. Moderating a busy subreddit becomes very cumbersome using the native tools, so even if they don't go dark we will see a dramatic decrease in browsing quality as spam fills up the queue.
Maybe then, reddit will get off its ass and build proper tooling for moderators.