The Wii was a massive success, and it wasn't "short-lived" either as it's life span was very similar to the similar selling PS1. It's only short lived compared to the over-engineered and abnormally long-lived PS360.
I wouldn't say that Nintendo systems have been in decline since the NES with the exception of Wii; I would say each system has been a unique situation with varying outside factors. Nintendo products have been loved by many throughout the years, as systems like Gameboy and DS show. Declining market share for several systems is each for a different reason: SNES (launched very late after Sega) N64 (no Disc drive, again launched late), Gamecube ( a total disaster in concept). Wii, for the first time ever, launched the same time as Sony's product and was a mature looking system without an obvious flaw, and the market responded accordingly.
The jury's out with Wii U, but I see its situation to be nothing like the challenges that faced N64 or Gamecube. it may struggle if 3rd party support is weak, but it may have great success even in spite of that if the 1st party software is strong because of it having launched first and with the lowest price.
