The idea seems to be that multiplayer games allow players to keep playing indefinitely

Not really. Games only have so much content. Multiple people will get bored of that content eventually just like a single person will in another game if the content doesn't have something engaging. In fact, I would argue it's the opposite. Multiplayer games by design have a very short lifespan. Multiplayer games ultimately lack a lot of content because they rely on the players to create an experience for each other and developers don't want players to play indefinitely, they want them to buy the next game the following year and move on. Even if we give developers some faith, people still eventually get bored. DLC exists for both multiplayer and single-player after all. It's not a problem exclusive to one or the other. Single player is endangered not because it isn't engaging, but developers are AFRAID of creating a satisfying experience. If games are satisfying, they have a longer lifespan and companies can't make games as fast and people will get burnt out on the franchise (see Assassin's Creed). With multiplayer games, they have the work of the community. No one plays the old multiplayer games anymore except for in LAN setups, there's not any content left because so much of it is driven by the community that people are FORCED to buy new multiplayer games, while a single player can still go back to the old Elder Scrolls, Fallout, or Witcher games and get as good of quality experience as they did the first time they played.

Yes it should... It's the best way to escape reality for a bit and have some quiet time for yourself. You won't get the kind of self therapy and treatment from Multiplayer Games. It just rouses you and a shit ton of other stress factors.