The current consoles are all nearly 3+ years old (PSP, DS, Wii, XBox 360, PS3). The DSi doesn't have better gaming capability, just a camera and microphone. Why would hardware sales be increasing when there's nothing new to buy?
Why do you think car companies come out with new and updated models every few years? If they didn't give consumers something newer and shinier, people would just stick with their old cars till they stopped running. Hell, Wii sales would probably increase again if Nintendo just released the black version here in the U.S. It doesn't even need better hardware, its just something new, and an excuse for many people to spend money. People like spending money, they just need better excuses in these tougher economic times.