I, on the other hand, much prefer being able to play an MMO solo and grouping with people when I decide to be social. Locking content behind grouping is just plain annoying. As with all games, I'm there to explore the world and the story and the less I have other humans involved in that, the better in most cases.
Probably why I actually leave multiplayer on in No Man's Sky. There's people around, stuff happens, but there is no need to engage if you aren't in the mood.
Plus, if you are feeling sociable and want to, you can go to the anomaly and team up for bonuses.
You can have multiplayer influenced worlds and environments, enjoy seeing what others do and create in that world, without enjoying the partying up aspect. MMOs with competitive markets, faction influencing, or competing with players in general rather than direct conflicts.
One can enjoy the world building of MMOs without wanting to be a part of a guild or raid parties.
Except there are many things that can be done without groups and more games add functionality to accomplish this (comparing FFXI at launch and today, for instance). Multiplayer could also simply refer to the fact that multiple people are playing this game at the same time, which would fit better with the first statement.
If a game wanted to force multiplayer, they should do it from the start and just not allow doing things solo. Then, you're right, I simply wouldn't play that game.
FFXIV lets you do almost all of the main story solo now, making the amazing story accessible to so many people. There are a few instances of having to do an alliance raid or trial every 40 hours of solo content or so, but if you weren’t ok with that, why even play an MMO? It’s in the name, “multiplayer.”