The thing you have to remember here is that how the Xbox 360 and PS3 are doing is completely irrelevant to Nintendo's investors.
What they care about is how Nintendo is doing relative to itself. If they are doing worse than they used to do, it's bad, because it means that the value of their shares are going to go down, and if they are staying stagnant, that is also bad, as the value of their shares are going to stay stagnant as well.
Due to this, Nintendo has a constant pressure on it to grow, and if it doesn't grow, it gets viewed negatively by investors, even if they're the most profitable gaming company, since the only thing that matters to investors is how their share price is changing.
Technically you can avert some of this pressure by giving out a dividend, but I highly doubt Nintendo is going to do that.
Of course, the thing that needs to grow is the company as a whole, so if the 3DS can grow them more than the Wii is shrinking, they will still be viewed well.