I've posted this before and got shat on all over by Rol and Viper. The Wii was not even considered next gen by its own leader (Hiroshi Yamauchi), so this is no surprise to me.
Gens are rooted in the concept of technical advancements requiring changes to a console, things that could not be done before. That isn't the case for the WiiU, wasn't the case for the Wii.
A gen is defined by machines that compete in some way in the same market over the most time of their life. The most important factor is, that the machines are released in the same timeframe.