I don't think Nintendo will ever leave the console market, they have had the best business strategy since they entered video games.
They know how to turn a profit from their systems, even though the gamecube can be considered a failure from a gamers viewpoint, Nintendo still made tons of cash. To think people actually thought the Gamecube would be their last console is pretty funny.
Sony will make another console, it was very important that the PS3 lost this gen, Sony needed to be knocked down from their podium otherwise they wouldn't have to rethink their strategy and would continue pumping out the same crap over and over again. It happened to Nintendo and they took gaming to a new level with Wii / DS, will sony do the same in a few generations?
Microsoft will continue to support the xbox if it turns out money this gen. A 3rd system is almost guaranteed but a 4th would only happen if microsoft starts gaining money from the systems.