I don't even own any of the new generation consoles yet so that should be a good indicator of how much life they have left in them. I'd say a minimum of four more years.
There is literally nothing that I can think of to warrant upgrading. Not a huge graphical increase. Online functionality and high definition have already been fully tapped with the Xbox 360 and PS3, which was this generation's gimmick that warranted a new round of systems.