The definition of MMO has been slowly watered down over time. Let me make this clear, if the majority of your game takes place in personalized instances, IT IS NOT A FUCKING MMO. So many people have tried to posit every live-service game as an MMO and it drives me up a fucking wall. According to these mongs, Destiny, World of Tanks and War Thunder are all MMOs. Do they have big persistent worlds or any ither definitive traits of an MMO beyond progression tracking and marginal social lobbies? NOPE.
The only excuse you have in an MMO for separating your players from each other is if it's 2006 and you're Blizzard and you just have so many nerdlings signing up for WoW you absolutely HAVE to give them their own realms or they'll crash the server. Anything shy of that reason, you're a bitch developer and your game is not an MMO. I'm unironically less than 5 years from some zoomie zoom trying to convince me Call of Duty is an MMO because progression between matches is tracked.