Nintendo has been consistently popular throughout the years and.. even in its low sales points, still has managed to make a profit. Nintendo, much moreso than the others, has a very loyal fanbase who will likely never stop buying its games. Even people who have sat out one of two generations are now getting Wiis. People who owned an NES back in the 1980s are buying Wiis for their kids. The only way to get loyalty like that is to always put out great games and most people will agree that Nintendo does. It will likely be around for years and years to come.
Its really hard to tell with Sony, now that PS3 sales are finally starting to turn around. They obviously have lots of financial resources and can likely ride out rough patches in sales. The PS3 and Blu-ray will mutually ensure sales for the time being.
Microsoft also has nearly infinite financial resources but its shareholders are obviously nervous about its gaming division, and rightly so. XBox 360 is now last in sales in all three major territories and doesn't appear to be picking up much steam even with another price drop in Europe. Prior to the release of the XBox, Microsoft released some very successful PC games -- most notably Flight Simulator and Age of Empires. It is plausable that Microsoft could revert back to PC gaming when its investors say enough is enough with the Xbox brand.
