If sales for the PS3 and XBox 360 continue at the rate they have been, it will be really difficult for these platforms to have long lives mainly because of third party support. Allow me to explain ...
Everyone knows that developing a PS3 or XBox 360 game is amazingly expensive mainly because of how expensive it is to create the artistic assets for the games; this should get less expensive throughout the generation but will stay in the same (basic) range. Most third party publishers justify these development costs because they can release a game to the PS3, XBox 360 and PC for (basically) the same price.
This causes a problem that many people do not realize ... The PS3 and XBox 360 can only continue to see strong third party support as long as both consoles are their manufacturer's main console, and they are powerful enough to receive PC ports. For the first 12 to 18 months of their lives these consoles can remain in the recomended hardware range for most PC games, for the next 18 to 24 months they will fall towards the minimum requirements range, and after that they will have difficulty producing the same game for these systems as well as the PC. (This is one of the main reasons the XBox saw so little support in the second half of 2005 through 2006).
Now, in 2010 or 2011 the PC games will really start to disapear from these consoles libraries which will impact their hardware sales. At this point in time Microsoft or Sony will start looking at how they can release new hardware, recapture these developers attention, and get a jump start on building their next generation userbase. When one of these consoles essentially leaves the market the userbase of the other system has to be large enough to justify exclusive development.
This argument (of course) is assuming that neither of the systems ends up begining to sell at a drastically faster rate than it already has been.
