I think we've reached a point where we can say, without any hesitation, that everything is an mmo. GTA5? mmo, because it has an online mode. Raft? That's an mmo, because people can see other people's rafts at the login screen. Total War games? Totally mmos, because there are multiplayer battles that a lot of people participate in. Uno? That's an mmo. I mean, you can't play it alone. Pogostuck? Another great mmo. I can see all the other players who are playing it at the same time as me.
These are all mmos. I think we can all agree on that.
Some incarnations of GTA5 might be MMOs. There are persistent servers that accept and react to contributions from far more players than my arbitrary cut off, but this is not the standard state of the base game.
I wouldn't consider any of your other examples to be MMOs, even if I unequivocally consider
Elite: Dangerous to be one.
You need to define MMO. My definition is the in-game ability to interact with a number of other players past a certain threshold. My entirely arbitrary threshold will be 256 because I played on a lot of 256 player Tribes 2 servers back in the day, and that didn't quite feel MMO in a match-based tactical FPS with no persistent worlds.
Elite: Dangerous is an MMO because I actually cannot play it, outside of the tutorial or training missions, without playing with many thousands of others, via the shared, persistent, setting. Even without that shared setting, there have been many occasions where my CMDR has encountered
way more than 256 other discrete CMDRs in a single play session. That's difficult to accomplish in the current game, but it used to be fairly common. Regardless, the BGS is a huge part of the game; everyone influencing it any time they do anything means that I regularly play with
all active players, even if that play is heavily abstracted.
Something like Dungeons & Dragons Online certainly
can be an MMO, but I can and have played it as a near single-player game for years at a time. I can flatly ignore all other players, as it's a theme park game and I do not need to have my character group with anyone. I can select quest difficulty sufficient for whatever single character I bring and experience the same rides on this theme park as anyone else. Only a tiny handful of content is categorically impossible to do alone, and it doesn't need to be done.
I expect everyone to have their own definitions and cut offs, but even assuming the mean definitions commonly citied, I would not hesitate to describe Elite: Dangerous as an MMO and most of the titles you refer to as not MMOs.
That's cool. Like how all games are RPGs, 'cos you play a role in all of them.
Not all games presume or account for role other than 'player'. It's a lot easier to role-play in
Counter-Strike, or
Flight Simulator than in
Tetris. I mean, you can pretend to be a block, or something that for some reason has to order blocks, but it's a real stretch compared to any game that explicitly presents one with a character/avatar to play (a terrorist in CS, for example) and a relatable scenario for one to play in.