Nobody argued that the Dreamcast kicked off the 6th generation, as its capabilities were far beyond what the 5th gen Playstation/Saturn/N64 could achieve. However, the Xbox console, two years later, was much more powerful than the Dreamcast, had better online and even an HDD. Should we consider Xbox part of gen 7? No.
If Sony decided to go with a GTX Titan GPU in the PS4, would that suddenly be considered a 9th generation machine? Again, no.
Furthermore, technical power is still only part of the equation when it comes to a next-gen experience. The WiiU offers asymmetrical gameplay and the ability for off TV play with select games right out of the box for everyone who owns one - something that's never been done before and something that the PS4/XB1 can't compete with from a design standpoint because Microsoft isn't offering a streaming-screen gamepad and Sony isn't packaging a Vita with every PS4. So, riddle me this: Should PS4/XB1 be relegated to just really powerful last gen consoles? The answer remains the same. No.
So face it: Yes, the WiiU is indeed a next gen console. The PS4 and XB1 just happen to be more powerful next gen consoles.