It can stay around for as long as any typical generation, just the same as Gamecube or Xbox could. And people enjoyed them plenty, and had plenty of games to make a case for their system of choice.
And at least in the case of PS3, not only is it extremely powerful, but it also has a factually superior online system to the winning console of this new generation, and many other features to be proud of. So I think it's in the best interest of Sony fans to accept the system as a sales failure, and get comfortable in the third place shoes. After all, third place doesn't mean bad system...