Look - I don't want to get into the next gen/previous gen arguments.
It's not, and HAS NEVER been about system power - more about release time. The Wii's graphics looked closer to GC/Gen 6 than the PS3/360/Gen7 - and that means exactly jack diddly squat when discussing what generation it is actually part of . Going on 30 plus years now. This doesn't change because a few forum goers want to have a swipe at Nintendo.
You know that uncle who's 15 years older, but actually looks younger than most people his age, even to the point of looking as young as people 15 years his junior ? What generation he belongs to isn't decided on appearance.
I don't think most Wii U graphics are that much better than 360/PS3 graphics - but it's next gen/Gen8. Indisputably. And also the only console maker on the market to have been releasing a home console all through Gen 3 (NES), Gen 4 (SNES), Gen 5 (N64), Gen 6 (GC), and Gen 7 (Wii).