I am nothing if not a pessimist at heart. I tend to think that most game companies have decided to cater to gamers much like a pusher to an addict. Especially those with generally successful product lines. The basic attitude being that gamers will whine, and form impotent petitions, but will never actually boycott products. Typically performing their whining after having puchused their wares.
As such, adding a lot of replayability to certain kinds of games defeats the purpose. After all if you've got a big umbrella company with several differant subdivisions all of which produce shooters or whatever, it's counter productive to produce a BIG shooter with a long campaign that could take a hundred hours to beat, when you could say produce one with a dozen hours of gameplay (plus multiplayer), which the rube... err customer, will finish just in time to buy the NEXT shooter to get his "fix". The result being that you've got these guys who might go through a dozen or more short games a year.
Now, there are exceptions, which is why I said replayability is not dead. For example most RPGs or pseudo-RPGs set themselves up specifically to have a "new game plus" mode or whatever with additional content or bonuses for at least a second playthrough. Most action games however rely on a multiplayer option to add perceived value, but for the most part such multiplayer usually receives a lot of complaints with most serious competitive players flocking to a handfull of titles. Your typical shooter has people complain about the multiplayer as being tacked on, or give praise simply for it not being "totally noisome" and stick with it just long enough to max the trophies and achievements (perhaps through boosting) and then move on.
Such are my thoughts on the situation. In general replayable games are not dead, but limited to generes like RPGs or Strategy titles for the most part, both of which are produced in comparitive numbers.
Maybe this will change at some point, but in general I've found people tend to complain about game length and replayability, but rarely if ever pass on a title because of it. Even when people do, it doesn't happen in great enough numbers to really influance the industry.
But tgen again, as I said to begin with, I am a cynical and pessimistic person.