I don't think RPGs are too long. In fact, the longer the better provided they don't reuse crap just to artificially create the perception of long-ness (DA2, anyone?). The CoD franchise can get away with charging $60+ for a 4-hour game, but that price is all about multiplayer and replayability (if that's your bag). An RPG gets 1 run through from me, with a second only after a few years has passed and only if it was good. If an RPG isn't at least 40 hours of quality material, it's disappointing to me and not likely worth what I likely paid.
That said, it's well known that the vast (I mean, the vast) majority of gamers never see the end of games, period, let alone the end of long games. As a writer, this would have to be frustrating. Why save your best work for the end of the game knowing that 1-in-a-gazillion or so people who play it will actually see it? More importantly, as a studio, why spend equal money on end-game content when you can front all of your budget for the first one or two acts, also realizing that's about as far as most reviewers will get before his/her article is due?
Perfect example of this is the Witcher 2. Act 1 was brilliant and took wonderfully forever to complete. Act 2 was still fairly good, but was a tad shorter. Act 3 took like 10 minutes, felt rushed, and was almost an afterthought in an otherwise excellent game. Either the studio ran out of money toward the end, or the writers ran out of ambition, or a combination of both.