I’m not sure that a persistent world is required to call a game an MMO.
What if there was a game like battlefield for example… but with larger maps and with massive 1000 vs 1000 player battles that would only last for a few hours (I know that’s kinda crazy… but let’s just say that exists for the sake of the argument).
Wouldn’t you say that’s some kind of MMO?
To me it seems like it completely depends on how many players you can potentially see (and interact with) on your screen at the same time in the same world. If it’s under 100 then I personally wouldn’t call it an MMO.