I was thinking if an instance owner cannot afford to keep upgrading servers, it could close its doors to new registrations. This will create a need for more instances and users will be more evenly distributed.
But it's my understanding that resource usage will keep increasing even if you keep the number of users constant on a server (as more content is posted in the communities that users are subscribed to, and that content needs to be synced and stored).
If anything, this is the month where we'll find out... A lot of medium/large instances have made the jump to dedi servers and keeping those up isn't cheap