I don't see Microsoft, Sony or Nitendo leaving anytime soon, all 3 of them will compete in the next generation of consoles, why? Because they can.
Sony won't leave because: PS1 was a success, PS2 was a success, PS3 managed to push blu-ray, and as a game console isn't a total failure.
Nitendo won't leave because: they've been here for what seems like forever, Wii is a huge financial success, Mario, Zelda and Donkey can still make $, Nitendo is a core gaming company, it's their business, and livelyhood.
Microsoft won't leave because: Xbox 360 is a huge success compared to the original Xbox, Xbox live is a huge success, they're currently making profit, and will likely have made a good profit by the time Xbox 360 stops producing, they've no reason to leave.
