Nintendo is the best game developer their is, and they make all their games exclusively for the Cube (go figure), what games has Sony and M$ made themselves? If all third parties left Nintendo, they would still make money with their own dev. teams. If all third parties left Sony or M$, they are out of the gaming business. On the other hand, Sony and M$ can lose money on the gaming market and make it back with their other business products.
Also M$ and Sony are looking to make their system the all in one home theater hub, whereas Nintendo wants to make a gaming machine.
And also Nintendo makes a lot of money, and they have a lot of money in the bank, and gaming is the only thing they do. With the next generation, all three systems will be able to display basically the same realistic graffics, M$ and Sony can hype all the specs they want, but the bottom line is that the game quality is what will be the main focus of the next consoles, And as I have said, Nintendo makes the best games, and dont need third parties to succeed (third parties will help them, but the lack of them will not break them).
Nintendo is doing just fine, and will continue to do just fine for a long time.