I agree Nintendo will not force developers to support HD, but Nintendo will always support it on the Wii 2 and the best developers will as well. Any games us "hardcore" gamers want to play will support it... with the possible exception of Wii-Ware stuff.
Concerning Wii vs. Xbox: you can say it is more powerful except it has a "different" (read: inferior) shader model. That's a pretty important difference. Obviously technology needs to be paired with good art, but developers are definitely figuring out how to make shaders look really good now.
So a big question for the Wii 2 is will Nintendo make the switch to the modern shader model? You would think that would be a good idea now that most developers are experienced with it, but to use it properly ups the art costs dramatically. Meanwhile, they need to worry about backward compatibility as well (I'm sure they'd love to continue selling the same Wii-Ware and Virtual Console titles).
In any case, it's practically certain that whatever the Wii 2 is, it will quickly (if not immediately) be far out-classed by the NextBox and the PS4 whenever they arrive, and whatever power it has will go largely unused. Basically, the same situation you have with the Wii now.