ICStats on 01 August 2014
Wii U is in it's 3rd calendar year, so compare GC in 2003 when it was selling almost double the Wii U's estimate for 2014.
GC peaked in 2003.
In 2004 GC started to decline, as PS2 was dominant and developers stopped supporting GC as much.
We don't yet know if Wii U will be similar, but there's little reason to be optimistic right now.
My 8th gen collection
