I agree.
Not every game is made for every type of person. Some games being multiplayer only are okay. Just like some games being single player only are okay.
Since Overwatch is the trigger here the biggest thing I'll say is that the level of content is perfectly fine. The game is fine and very replayable. It's extremely easy to rack up hours of game time with the base game maps and characters. The kicker that pushes it even further the edge is that Blizzard has committed to releasing future maps, modes, and heroes for no additional cost provided you paid the $40 or $60 to buy the game in the first place. And seeing how Blizzard has supported several of their other games literally well over a decade after their release, it makes a lot of sense why this is a great value. Extremely fun and addictive gameplay that leads to lots of replay now, with even more content coming later for free. Can't really knock that.
If you want a single player campaign, maybe that will be a separate sort of game in the future, but that isn't this game. And that's okay. It doesn't have to be.
The difference when it comes to, say, something like Star Wars Battlefront is that game had even less than Overwatch at launch, isn't as replayable, forces you to pay $50 if you want the planned additional content, and essentially already has confirmed loss of support within a year and a half due to a sequel.
It's a case by case basis. In Overwatch's case it's fine considering the circumstances. Personally, I do typically spend way more time with multiplayer only or multiplayer focused games than single player only or single player games. Most are going to run you 8-20 hours or so. Some RPGs will be longer than that, maybe up to around 100 depending on the game. Multiplayer focused games like Overwatch you can easily put well over 100 hours in without even noticing.