Cost is always an issue. I'm not going to pay $60 for 5 hours of entertainment. Period.
Luckily, people with intelligence can actually wait for sales and whatnot, but it's a dangerous precedent to set for developers and especially publishers.
Plus, as someone who actually appreciates depth and substance and not just carbon-copies of every game ever made, I'd rather buy less games and have them be substantive than go through four or more 5 hour games a month.
No, cost is not always an issue. For some people $60 is insignificant amount of money spent easily on a meal that is less than 5 hours.
There is no reason you can't have depth and substance in 5 hours. Length != depth and length != substance.
I am fairly tired of the idea of hours/$ as a measure of value of a game. I realize if you have free time but less money that is a great measure but otherwise it is as useless as measuring polygons on the screen for entertainment value.
I like long games but I also like short games. What I want are good games, good for 5 hours or good for 20 hours I don't care. If you take a good 5 hour game and make it 10 to satisfy people that think hours/$ is the right measure you dilute the quality. It is no different than a movie that is overlong.
As a person that appreciates good games I'd rather play four good games a month than one good game a month. Unless you only play cookie cutter games that should be four different good experiences as opposed to one.