Why is a high-def Wii needed?

[Reasonable]

I guess the only real reason I can see is that, eventually, SD TV will be gone so any console will need to output at a resolution suited to minimum HD TV - not sure that affects Wii.

Also, I don't think there will be a Wii HD - I think that's the wrong way to look at it. Each Nintendo console has seemed distinct to me, each with its own name and style, so I rather take the view that at some point Nintendo will release an HD console, the question is whether it will be their next one or the one after.