So, reddit communities aren't going "poof", they'll still be around. However, /u/spez's actions will cause users to migrate to other communities, such as those hosted on Kbin. The short term effect will be fractured communities.
I imagine something similar would happen if a fediverse admin pulled something similar. Users that don't approve would move to another instance, fracturing the community. The advantage of the fediverse is that creating a similar community on another instance is much easier than going from reddit to a completely different platform. This gives users more bargaining power, making unpopular decisions by admins less likely.
I imagine hosting these instances isn't cheap, or at least won't be if many users migrate here. Some servers will opt to monetize in some way to pay for the operating costs. I think one of the exciting things about the fediverse is that instances will likely be subject to natural selection with respect to their operating costs and monetization strategies. In the long run, only the instances that can pay for their operating costs, while still being attractive to its users will survive.