To emphasize, I'm not saying anything about the console as a whole, nor about those who make it their system of choice; I'm speaking solely from the gameplay perspective of Rock Band on the Wii as compared to the PS3 and 360.
Meanwhile, RBN 2.0 is right around the corner, with authoring teams already provided the software and keys and harmonies RBN songs rapidly approaching. That's growth right there, saying to me that everything's still moving strong. Again, sadly, not for the Wii in this one area, but just because a creek bed dries up doesn't mean the river isn't raging.
Some people are of course going to take any change as a negative and their sky will always be falling. For me, as unfortunate as this was for our Wii friends, I've yet to see anything to convince me that there's anything but bigger and better coming our way.