Eh, I'm not sure I agree with that particular point, as all that adding HD probably would have done on the Wii would be to jack up the price tag on the console which would of had a impact on the demographic that mostly bought the console, let's be honest those that were playing the motion control Wii games probably didn't care whether it was HD or not. I don't think HD would have made the Wii sell anymore then it already did which was alot.
The Wii's success is a complicated thing and part of what plays into what's going on now. Nintendo has had a back and forth history with their innovation with their consoles working for and against them at various times. Sometimes it works great for them and other times it simply doesn't.
In the Wii it did work out for them, even though the software sales themselves weren't extoridary, Nintendo had money on each console they sold which means doing more or less their own thing brought them more money in then they ever had with the Nintendo 64 and Gamecube which were more mainstream in their design and intention. The Wii became a well known name for better or for worse. So they banked on the Wii brand and introduced the tablet-like controller as a new innovation on top what techology they had developed during the Wii cycle. However it didn't work, mostly because the audience they had created with the Wii wasn't interested in the new console or didn't even know it was a new console and Nintendo couldn't get enough 3rd parties on board to support the thing which gave it a reputation of having no games, which ironically could only be fixed by 3rd parties putting out more games which will only happen if they actually move the console, which requires more games the Nintendo can physically put out on their own.
Even if the Wii U suddenly gets a whole bunch of PS4 or Xbox1 features and sold a lare numbers, I'm not really sure it would actually save the situation because it seems like it's been mostly written off at this point by most developers and consumers as a viable option. I suppose a lot at this point depends on E3 but if some consoles don't get moved this year, I'm pretty sure the Wii U is more or less heading for a brick wall.