People use these systems as public spaces in which they can build communities
What really amused me is when Reddit sent messages to moderators saying things like "your subreddits are public spaces which users depend on, so you should open them".
When obviously what they mean is "your decision to close your subreddit is hurting our revenues, and if I don't keep up the payments on my BMW then I'll be forced to drive an old Honda, so either open the subreddit or we will forcibly do it ourselves".
Would it be easy to find new moderators though? Who would want to moderate for Reddit at this stage? I think most of the people who would show any interest in moderating are currently too angry with Reddit to want to do it.