Thought on this a bit more, and I’m thinking encouraging users not to silo (and make it easy to discover instances and new communities) will probably be the best bet for scaling the network long-term.
“Smart” rate limiting of new accounts and activity per-instance might help with this organically. If a user is told that the instance they’re posting on is too active to create a new post, or too active to accept new users, and then is given alternatives, they might not outright leave.