I don't feel like there are a lot of co-op games anymore. Back in the day, every game was co-op. Just got less and less over the years. I may have a theory about this though. Brace yourselves
The demographic of the average gamer is in their mid-20s to 30s (according to something I read I don't feel like Googling right now) with a steady income. Not unlike everyone here. So going by this demographic, back in the days of Genesis, that would have made us kids. We had a lot of friends who came over for play dates; so what would we do? Play a little Street Fighter or Mortal Kombat and have a great time. PSX came out, same thing. Still kids, still had play dates, so co-op games were the way to go.
PS2/Xbox/GC came out; adolescence age for the average gamer, maybe college for the higher end of this sample demographic. You had friends but it was mostly in your own little clique. So you'd have the most popular co-op game for when they would come over your house after school or to your dorm to play a little Guitar Hero or Super Smash or Mario Cart. But that was also the time online gaming started taking a foothold.
PS3 came out, and it was time for adulthood. You are in the real world, have a job. Most likely a crappy one. You go to work and go home. You may have a girlfriend if you're lucky. You'd see your friends once in a while and you may have LittleBigPlanet when you see them, but most of the time it's single player. Maybe a multiplayer game if a buddy is on PSN or someone you met while playing online. Now the PS4 is out. You are the shell of your former self most likely sitting alone in a dimly lit room and a Jack and Coke sitting on your coffee table. You may have one friend you see once in a while. There is the one co-op game on your shelf collecting dust that came out somewhat recently and you bought it just to have a little hope. This game is most likely the newest incarnation of a co-op game you played with your friends back in the PSX/N64 generation just so you can relive a smidgen of your youth. But it doesn't. Most of the time you are on Call of Duty trash talking 10 year olds and noobs in a pathetic attempt to justify your miserable existence.
Anyway, just a theory. I could also be full of crap and the industry is just trying to get you to pay for multiplayer games
