Sorry that I don't have an answer to your direct question, but
It doesn't necessarily need to be at a large scale, because of federation. The technical knowledge needed to admin an instance is a barrier to entry now, but probably can be improved. We could eventually see an ecosystem where your WoW guild (or your college buds, or your found family, or your fantasy football league, or equivalent smaller community) hosts an instance for its members. You can still participate in federated discussions, and the subscriptions of the instance could stay comparatively filtered to what's most important to the users of the instance.
You'll always have bigger generalist instances, but the flexibility you can have with really small and topical ones shouldn't be forgotten about imo, especially as the platforms/technologies mature.