Third parties decide when they make their next gen engines.
Game engines are typically very scalable, though. A PC game made with a 2012 GPU in mind can be scaled down to run on a PS3 or 360, for instance. As long as there's support for programmable shaders in the hardware and enough RAM (ie. not Wii), engines can be ported. Even Unreal Engine 4 will make its way onto iOS devices, according to Epic (in a limited form, of course). Next generation consoles will have some nominal improvements in their shader hardware and be able to pull off some nice tricks, but it's nowhere near the visual leap we saw compared to systems with none at all (PS2, GC, DC). Regardless of the brute force power of the hardware, unless publishers are afraid of money, they will make sure to port their games wherever they can. Multi-platform is pretty much the name of the game anymore.