So next gen begins in 2016? I'd be willing to take my first ever bet on VGChartz that either Microsoft, Sony or Nintendo will release a new system before that.
Nintendo will likely release before that, but imo its a mistake to think that this forces Sony and Microsoft to jump with N. the reason is because whatever N pushes next could likely be less powerful than the PS3 and 360, so that it can compete on the same price scale.
thus Sony and Microsoft might decide not to release new consoles either, because the install bases of their current consoles are imo too low to suggest a new console would reach the same level of success. considering the amount of money both companies have spent on hardware, software, manufacturing, manpower, etc.
not to mention its expensive for current gen developers to work with the existing tech, it puts both in a very veneruble position if they choose to release again soon.
my theory is that PS3 and 360 will remain on the market about 4 - 5 years, Microsoft i'm not so sure, as their counting on the success of Kinect, which imo isnt enought to prolong another 5 years, Sony are in the perfect position to usurp Nintendo should they release again, because they will have the advantage of lead platform development, motion control, blu ray etc, and imo neither Microsoft or Sony want to slug it out but cutting off their current consoles and release again with the same sceneario in the next 2 years. both have worked too hard imo to abandon ship and start again.