I don't think it was intended to be Nintendo's last console. I think they knew they had to do something different so they gave it almost all they had by putting as many features and as much of their talent as they felt viable for a low cost. The low cost and odd marketing (name Wii, simplistic design, Wii Channels, Miis) show that they had actually a plan to take over secretly, but as with the N64 controller, making it both a traditional AND new controller form, they didn't know what to expect and had to consider "outcome B".
If this console were to be a HUGE success, it needed as much leverage as possible.
If this console were to be a HUGE failure, then they wanted to go out with a bang.
Either way, they were setting themselves up for what they thought would be an awesome and fun ride. It wasn't until E3 2006, that they knew that they were headed toward the first of the two. Now that they have that position though, I don't know if they know what to do anymore. They are stunned and I'm afraid they are going to go back to what they used to do which is rely on tried and true franchises with very little surprises. It's the surprises that got them to where they are right now! I guess we'll see.