VGKing on 12 December 2012
Next-Generation starts when there is competition.
For example with the handhelds, next-gen didn't truly start until the Vita launched.
A generation is defined by a bunch of competing products. If there is no competition, there is no new generation. If Sony and Microsoft were to let Wii U have 2 years on the market we could consider the Wii U an extension of 7th generation instead of part of the 8th.
Just my opinion, I'm sure many will disagree.
