Consoles life expectancy are around 5-6 years. This generation is no different. In a few years Microsoft will release a console that will destroy the PS3 in hardware specifications and developer friendliness. If Sony thinks they can sit on the PS3 when that system is out they are going to be making a big mistake. It is more likely there will be a ps4 to compete with the xbox 720 or whatever they want to call it when it comes out.
PS2 had a good run. It was released in 2000 and will be discontinued at the end of this year I believe. When they released the PS2 industry standards gave it 5 years or so. By the end of this year they will discontinue the PS2, so that was a solid run. The PS3's potential hasn't really been tapped until now with the release of MGS4. It's logical to assume that the cell processor wasn't used as efficiently as it will be with future games. Figure in a year or two the system will be much easier to make games for and the catalog will get much better.
At this point Microsoft will be rushing to put something out to compete more effectively with the PS3, most likely surpassing it. The issue has always been just that, they rush. Rushed systems are at the butt of every joke and I think you know what I mean. Naturally both Sony and Microsoft are surely already building prototypes to the next generation systems, tinkering if you will. Sony seems to always come out on top with well integrated machines that give out excellent performance no matter what. You must admit that the original XBOX and XBOX 360 fall short in terms of hardware integration and reliability compared to Nintendo and Sonys' systems.
You also have to consider that like every new consule release people are bent out of shape about buying a new system when they feel like their old system is still very cutting edge (especially when the new system costs 3 times what their old system costs at the time).
Sony does need to get more exclusive titles under their belt.
And of course games like Far Cry 2 being released on the PS3 platform can't hurt either.
We are agreed in that Sony will release a competing system to Microsoft in the next generation around the same time period. It's also likely that Sony will not release a system until they have looked far into the future in regards to what they will need or want the system to offer for the present and future.
Edit: I just realized that Far Cry 2 is a multiplatform release. This may not be as relevant as I thought. I guess we will see how each system looks when the game arrives.