One trick with this subject is that if you ping gaming press and a lot of 25-35 year old young urban technology geeks, I think you may find a slanted answer to "should games be shorter".
The press is full of people who can't even be bothered to finish a game before reviewing it.
Young geeks are harried, pressed for time, and sometimes the victims of their own busybusy lifestyle, so they want everything to be mobile, shorter, faster, more instant, and over with so they can rush on to the next task at hand.
This is not theoretical, I've sat and listened to plenty of people in these positions say "Oh, I wish every game was just 2 hours like a movie, who actually cares about playing a game for more than a few hours? It's a waste of time."
But games are not movies, or books. The interactive part, the actual playing-of-the-game is an experience that is its own purpose. It should be enjoyable for its own sake, even if the game has a story and a specific ending to the narrative.
Some games are padded to increase length, and that can be bad; but with the attitude of many modern game players, even what constitutes "padding" is up for debate. For some people, if you ever do anything remotely like something you did before, it's "padding" and their attention span is so short they get mad and quit the game right there.