First of all, I'd be willing to bet that Nintendo would rather go under than be bought by an American company, even if the japanese regulatory system allowed it.
Second of all, Nintendo is very, very profitable. They have been consistently profitable, every single year (as far as I know) since the NES was released. They were successful makers of playing cards, board games, etc for decades and decades before making video games. They have no reason to allow anything but the most hostile of takeovers, and Microsoft would have to pay an absolute crapton of money in order to do that.
If Nintendo's next two console offerings are failures that make the Jaguar look like the PS2, then maybe in five years. Speculating about 20 years from now is dumb and pointless.