I’m talking about 10,000+ players. Where you need clusters of servers and synchronization techniques.
There have been some small multiplayer games that have tried to pass as mmo’s but without the player base in the 10k+ range, you never encounter certain classes of engineering problems.
Your "synchronization techniques" pale in the face of destructible terrain.
Going by my logic, there are MOG’s and there are MMOs. Guild Wars 2, World of Warcraft, EverQuest 2, games where thousands of people can be on screen at once, where that server instance is synchronized with the rest of the cluster, for one seamless virtual experience. I’d say half of the self-proclaimed mmos ever really reach massively-multiplayer status. Games like Dota 2 and CS2 are played by millions but I wouldn’t say it’s an MMO because each match it’s 5v5.
Realm vs Realm or World vs World mechanics explain my perspective perfectly.
The original release of World of Warcraft had a limit of a few thousand per server too. It's only much later that this got increased. In the classic MMO era there's really only EVE Online that pushed to 10k and beyond, and never in the same star system. Single star system was limited to around 500 people for the game to still be playable. It's only later that they added time dilation which allowed for thousands to be in a single system at once.