The Wii is built out of spare Gamecube parts. Back when the GC was being slapped arround by the PS2, the market wsn't as big as it is now. Hench the parts are made in smaller factories. Nobody expected that they had to use the same factories for 7 years because their R&D team came up with nothing more then "stick waggling" and "guilt-trick people into playing crap because it's healthy" whereas Sony fought(and won) the HD-DVD/Blu-Ray war and Microsoft took a valuable lesson about how to not weld parts together.
Yes, they might be selling more consoles at the time. But what happens when they reach market saturation? They will be dead in the water while Microsoft and Sony slowly trudge along, fighting between themselves. The Wii severely lacks staying power and there's only X ammount of plastic toys people will buy, so they have to come with something new at one point in the future, and at the moment, they don't show any signs that they are preparing for that. Sony decided to entrench themselves and sit out the 8th generation, something that doesn't seem to work as of now. Meanwhile, Microsoft will have to decide where their loyalty lies, Windows 7 or their emergency response to the looming collosus at the horizon called Sony, a collosus that at a closer look, appears to have faceplanted in the dirt. With no competition(Sony incapable and Nintendo uninterested), there is no need to further develop the console market and all resources will most likely be shifted back to the pc.