Makes sense not to rush..
The gaming community seem to be delighting or despairing of the Wii finally beginning to loose ground to the PS3/360, depending on their religious afflictions. For Nintendo, sitting on a very large pot of money, I should imagine that the current situation is largely irrelevant.
Instead they'll be looking at Natal, XBox Live and their own experiences with things like iPlayer to decide what the next gen Wii will do. It seems to me that jumping in with a Wii HD to draw out the lifespan of a machine that is increasingly being outshone by other platforms would be at best an interim measure and could have costs they'd rather avoid. Far better to wait a while and make the neWii able to distinguish itself.
For now, the biggest threat to Nintendo seems to be getting the content model right rather than the issue of 'legacy' hardware sales dropping. They've used most of their IP on the existing machine, so the next one will have to find something fresh to do before it will be a compelling upgrade. Nintendo haven't had a misstep like Home, but outside of Mario or Wii Fit they haven't had a success like XBox Live or the iPhone App Store either.
There's a lot to be said for avoiding the avalanche of dross that 'fire and forget' app services have ushered back (not seen since the early frenzy of the 8-bit computing era), but it seems difficult to sustain momentum of a system when you can only muster a handful of triple A products. A middle ground of community engagement in an environment that supports discovery of a wider range of content sounds promising but is hard to implement (cf. everything from Home, to Wii Channels and on to the fringes of Facebook and MySpace).