Anyone who say's Nintendo cannot fall is just silly. If Nintendo had lost both their console and their handheld market, then by default they would also lose their software market because no one has the hardware to play the software on. Nintendo could probably take 2 hit's, I'd imagine that even Sony and Microsoft could walk away with another one, but it doesn't make them unstoppable.
Any company can pull a stupid move, management may change, deadlines aren't met, lack of interest from the general public, over saturation and the list goes on. Nintendo only need a combination of a few big negatives and it could go under.
Nintendo have been lucky in the last two generation's, without the Gameboy they would have had a much harder time coming into the current generation. It's not like Nintendo never had financial problems, hell in 1964 their stock's dropped to 60 Yen and they nearly went under.
Anything could happen, we've seen plenty of multi billion dollar corporations disappear and Nintendo aren't shielded from failure.