Very much this. They way it was explained to me was akin to what you are saying, single login, jump servers whenever. In practice it is not that

I’m a shitty developer by all measures but I assume such a feature would be tremendously difficult to implement especially this late in the game. Maybe I’m wrong but I don’t think so. Like would my username have to be reserved across all instances? Would a new instance have to get some kind of list of known usernames? Where would that even come from? Who knows, not me, that’s for sure

However if it could be done I think that would be a huge selling point. “Imagine if your server started being a shitty place like facebook and you could just move to another without having to nuke your account” is a pretty big deal