Ten years of mainstream support is a pipedream. They may be able to continue selling the PS3 for ten years, but the PS4 will need to be in the spotlight long before then. It'll be a secondary market at best.
I hear a lot people say that there's not really anywhere to go as far as graphics are concerned. It looks "good enough" right now. Well, that's not true. Until computer generated graphics can mimic reality and look absolutely photo realistic, there's still places to go. Now, I'm not saying that I would necessarily trade that in for a unique art style, but it's naive to think that there's no advancement to be made. Even when we do hit that wall, processing power can then be dedicated to things that have been largely ignored, like better AI and physics. Remember, added power doesn't necessarily equate to more eye candy. There's plenty of stuff to utilize it for behind the scenes as well. Maybe today's graphics do look good enough, but it wouldn't hurt to have large, more open environments with hundreds of NPCs on screen at once... would it?