As more and more consoles are being sold, the console makers will generate more revenue from royalties on software sales. Since the Xbox 360 and PS3s are still selling at losses, the software sales are crucial to their success. The problem with Sony is that they are getting fierce competition from Xbox 360 in the HD market, and the Wii in general. This does not bode well with them. Their only hope is to slowly make gains in the growing gaming market to a point where they can make profit from software sales. It wont be anytime soon, so they can only take a long term approach. However, when that day comes no one can say. With the economic crisis things are looking even worse for Sony.
In the meantime Sony just cant go and throw millions of dollars in to developing a new console while their current one is losing money.
If the PS4 ever comes out it wont be for many, many years.