If Nintendo seriously considers 3D an essential part of the Wii successor then we're stuck with Gamecube 1.5 for, I'd guess, at least three more years.
I honestly think that as successful as the Wii has been, hanging on to it for too long will hurt Nintendo. The Wii has been an over-achiever since day one. I think replacing it should be a priority. It has too many very serious flaws to just hope that people don't notice them for a couple more years.