OP is wrong.
NES and SNES both had redesigns after their successors launched.
N64 and GC failed to "win" their generation and thus no redesign.
Basically the two consoles that were going to have much longer post-gen lives got redesigned so they could continue to cut costs and therefore offer really low prices to ensure it remained highly profitable as sales declined over the following years.
The other two were cut off shortly after their successors launched as Nintendo knew there was no longer a market for them as it was basically gone at that point anyways.
While Wii will sell for a few more years, it has already had a redesign... i.e. loss of GC backwards compatibility (controller ports and internal hardware) and that's the only change it needs to keep costs very low for it to remain highly profitable for the next few years as it slowly dies off.
NES redesign: (launched October 1993, SNES launched November 1990)
SNES redesign: (launched October 1997, N64 launched June 1996)
