The point is gamers don't finish single-player modes, regardless of length.
Portal was one of the games listed in one of the links I posted and a very large chunk of that game's player base didn't finish it. That game was wildly popular and can be completed in a little under two hours if you know what you are doing. The average new player who does finish it does so in four hours. It's not a big game, and yet people aren't completing it.
The criticism that many of the games I typed up in my above posts are longer single-player experiences than Longshot is valid, sure, but my point extends beyond long games. Players don't finish short games either.