In the past when you reached the limit of what the ps2 and ps1 where able to do and the features they packed didnt cut anymore you needed a new console like the ps2 that carried in a dvd player. But now with the power of updates and digital content who needs to upgrade the machine? we dont have a new standard like HD, only 3d tvs but even the ps3 with an update can handle that and the xbox said they can too, so why a new machine?
Besides MS and Sony lose money when they launch a new console, they start making money after a couple of years on games they sell, so the longer the console stays on the market the better for them and for us too if you think about it. Bring good games and updates thats the only thing we need.
There have been numerous articles stating that current consoles can last for another 10 years because not only do we have the power, but we have internet + updates. Sony and MS can add new features like the motion controllers without purchasing additional hardware(well you have to for the motion controllers but other stuff can be updated without hardware.). By adding features they can extend the life adapting to new gaming trends, this couldnt happen with the ps2 or with the ps1.