Non-id Software FPS games way back in the early days of PC gamin were always meant to be singleplayer. Wolf3D, Heretic, Duke3d, Shadow Warrior.
Quake 3: Arena was the first FPS that started the downfall of the SP FPS and the upswing of the MP FPS.
I understand that. Good single player games would easily make their money back if they are good. I think a lot of devs do make bad games on purpose, because the online crowd of idiots don't care about bad SP, they only care about online play.
I can't play a multiplayer game for thousands of hours, I can barely play one for 10 before I get bored of it. Except for the original Unreal Tournament. I put many many hours in that game.
I play games to be told a story. I just don't have any interest in going online and shooting at people with no driving purpose behind it. The only non-story SP games I play are racing games.
I agree with you that devs should focus on one aspect or the other. Either make an amazing single player game and it'll sell well, or make a multiplayer only game that I can ignore.
If BF4 didn't have a campaign, I wouldn't have paid even the $20 I did for it.
edit: Forgot to mention Eve Online, I have played that one off and on for the past 4 years or so, and STO. Those are the only online games I play, and STO is a basically a connected SP game.