When the 360 and the other new consoles came out, gamers still saw games released across the generations. There was Gun that came out on the 360 as well as the previous generation and was great no matter what system gamers picked it up on. The biggest draw for owners was that the graphics in the 360 version were better. Gamers also got Marvel: Ultimate Alliance on the Xbox as well as the 360. The draw for the older systems is now those are much easier to get a hold of vs. the versions on the PS3/360. There was also Hitman: Blood Money, another great game that was a lot of fun. The last generation systems also received several sports games. While the GameCube and the Xbox sold just over 20 million systems apiece, they were popular enough. Fans of the GameCube were lucky enough to get The Legend of Zelda: Twilight Princess as the system died a slow death.
The Playstation 2 continued to see support long after the PS3 came out. But compared to the other two consoles of its generation it was beyond successful as it sold 150 million units worldwide. It made great sense to continue supporting the PS2, it was cheap to develop for, the system dropped in price, and it has a large install base much like the 360 and PS3 have. Support for this ageless system continued to the end of 2012. The PS2 was around for 12 years, which is an incredibly long time. The PS2 received a lot of great games over the last 4 years, most of them were released from 2006 to about 2009, after that it was mostly sports games.
The biggest reason for continuing to support the 360 and PS3 is due to the large install base vs. the newer systems. The PS3 and 360 have been around for 6 plus years and have sold over 70 million systems a piece. This is a huge install base, not as big as the PS2 but big enough to make developers notice. It makes sense to continue to support these consoles at least for another year to make money. Gamers should see this again with the PS4 and the new 360.
One game has already been mentioned and this is Watch Dogs [Ed Note: Assassin's Creed: Black Flag will also be available for both current and new generation consoles]. The game is coming out for PS3 and the newer consoles. This is good for gamers hanging onto their PS3s and its great for the new PS4 owners. I would hope the same happens for Grand Theft Auto V and the The Last of Us. Who knows? Maybe there will be exclusive content for the new systems? Who knows how long the support will last, maybe a year to two years at the max. I would bet both the PS3 and 360 will eventually drop to under $100. Gamers will just have to see what happens in the next year or so.