I completely agree that there are certain games that are harder to create good demos for. Spec Ops is essentially a concept game. Unless they had been marketing the message, there would have been no way to make the gameplay actually entertaining. You can't market "you will feel bad about playing" as a positive for widespread appeal.
Actually SOTL demo should have been made with a huge cliffhanger.
The introduction to the gameplay stays. (the first part of the first level). It cuts in a fine moment and fast forwards the story. Now, the problem of the current demo is that it fast forwards too much. It even contains a huge spoiler.
I remember... the demo for Doom was amazing and convinced me to buy a game that I would not have otherwise bought. I guess it was one of those examples where a great game had a great demo.
Years later, having become a huge fan of id Software, you can imagine my enthusiasm when I got my hands on the Quake demo. ...And then my disappointment, when the demo was dull, seemingly unfinished, and generally terrible. I would've bought Quake, too... if not for the demo -- which convinced me to go play outside for a few years, instead. Most of my Doom2 deathmatch buddies agreed and likewise avoided Quake.
How a demo makes sales: Look at telltales: The Walking Dead. I was interested in it, but wasn't going to buy it when it first came out. Then during Halloween Xbox had a ad saying the entire first episode was free. Downloaded it, finished it, then instantly bought the rest of the entire game because of how good it was.