Nintendo's internal marketing data suggests that there are still 200 Million possible consumers for the Wii and DS.
Now I doubt they'll be able to get to every one of those consumers but I'm sure that over the next 5 - 10 years they'll get to as many as possible. Having said that, I couldn't predict the sales of any of the consoles without spending a fair bit of time on it so I'll leave that to you guys :P