With the 3DS on the doorstep I'm sure they'll be fine.
If Nintendo relies a lot on profit from hardware sales then this sort of thing is inevitable. These are mostly one-time purchases. What use is there in buying a second Wii? You can really only sell that once to each household. But there is no limit to the amount of games you can make and with each new game you have the potential market of every Wii owner.
When you talk about Wii sales dropping, it's no different then if Nintendo had ONE GAME. Imagine it was just Super Mario Galaxy. Would we talk about SMG sales dropping and how Nintendo is in trouble now? Like the Wii it's an item you only sell to someone once. They don't just have one game. That's the whole business model of videogames. But they only have one Wii. If they want hardware sales to continue to bring them the moolah then they have to eventually bring in a Wii successor. If they don't want to, fine, but then they have to adapt their business model to make a profit on games alone.