No one here has mentioned Eve Online yet, I find that weird. Every player in Eve plays on the single game server. All 21k current players are able to interact with one another at any given time. The most that were ever online at once was 63k back in 2013. A single system, Jita, routinely has 800+ people in it. That can get up to 1500 people on the weekends. Could you imagine 1500 people in Idyllshire?

Source

Megaservers are possible in a fast paced MMO, the game just has to be built from the ground up with that idea in mind. There is no way the PS3 could have handled that. PS4s and most low end PCs would have a hard time loading a zone with 500+ players in it. In the case of Eve, most of the population is clustered in a small percentage of zones and there are huge swaths of empty space. You would have the same thing in this game.

I don't feel this game needs a single mega server, but multiple balanced servers.