boy there sure are allot of people responding before reading the thread, i think there is a number of reasons why games stopped having 4 player offline co-op.
1. i know you said that it isn't a "sufficient reason", but it is, most people don't have 60 inch tvs, they have tvs in the 20s so for most of the consumer's playing 4 co-op would be a pain, like watching a movie on a iPod, and since developers have made it quite clear they wont spend any time trying to satisfy a minority of their fans, they didn't bother.
1½. most people don't have 4 controllers, of all the gamers and people whit just madden i have met in my life i have never seen anyone have more than 2 controllers, so unless its a planned event it wont be possible.
2. no, i don't think me listing tons of reasons will help, more and more publishers/developers yank up their prices, make shorter games and remove things that take them too long, if dedicated servers get over looked and games like Rogue Warrior get made there are no hope for 4 player co-op, and after being burned by rogue warrior i think we are facing another black era of gaming, but who knows, maybe when the third coming arrives 4 player co-op will be one of the things to rise again.