Usually console makers start planning the next iteration of their machine soon after they release the current version, if not while they are making/releasing the current version so they can confer with hardware manufacturers one where they see hardware in 3-5 years.
Are you sure about that? I find it a little difficult to believe. Please look at these release dates (earliest region listed)
Playstation: December 3, 1994
Playstation 2: March 4, 2000 (+ 5 years, 3 months)
Playstation 3: November 11, 2006 (+ 6 years, 8 months)
NES: July 15, 1983
SNES: November 21, 1990 (+ 7 years, 4 months)
N64: June 23, 1996 (+ 5 years, 7 months)
Gamecube: September 14, 2001 (+ 5 years, 3 months)
Wii: November 19, 2006 (+ 5 years, 2 months)
So the trend has been about 5 years between releases. Hardware changes too much in 5 years, even at the development level - you can't plan something that far in advance and if it takes you that long to complete it then your hardware will be too weak. We can't even imagine what hardware will be available 5 years from now, even if we are in close contact with those at the forefront of developing it (or are developing it ourselves).