I think there is a lot of discounting the processing power that is (probably) in the Wii U, while over-estimating the processing power that will (probably) be in Sony and Microsoft's next generation consoles, and an underestimation of Nintendo's ability to do some interesting and creative things with a new user interface.
On the first two points, many of the best looking HD console games (especially the early ones) were unable to maintain a steady frame-rate above 30fps while rendering at a resolution below 720p. To get similar levels of detail while rendering at a resolution of 1080p with at a frame-rate of 60fps (especially with a second display or 3D output) uses a lot more processing power than the HD console have. To get an increase in detail similar to the transition from the Dreamcast/PS2 to the XBox 360/PS3 while maintaining this output level will require more processing power than will be available from a single GPU for several years. Beyond that the perceived difference in graphical quality by increasing processing power is facing diminishing returns. When you combine all these factors it is unlikely that Sony or Microsoft will be able to produce and affordable system in the near future which will have nearly the perceived improvement in graphical quality that the HD consoles had over the Wii.
On the final point, Nintendo achieved remarkable success with the Wii and the Nintendo DS because they created a user interface that made gaming more accessable. With how used to touch screens the average person is getting the average person (probably) finds a touch screen far more accessable than a conventional controller; and the conventional elements in their tablet controller (probably) makes it far easier for most developers to figure out how to make conventional games for the Wii U than the Wii. This doesn't mean that Nintendo will achieve success with this strategy, but if they can create software that justifies the approach I can't see why they couldn't match or improve upon the Wii's success.







