An add-on simply to make the Wii "HD" probably won't happen, in my opinion. My point lives or dies depending on on what you consider the term "Wii HD" to mean. If you just mean the Wii exactly as it is now, but able to output it's games at 720p and higher - I don't see the point. You'd just notice the low polygon-models and low-res textures and stuff even more.
If by "Wii HD" you mean an upgrade to the console which renders it on a par with the 360/PS3, not just with output resolution, but everything... that's a big ask from a simple upgrade attachment. In my opinion, such a thing would have to be an entirely new console. Maintaining a double-tier of Wii systems, one of which is what we have now, and one of which is upgraded (via an attachment or just as a new model like DSi) would be a massive headache.
Not just for Nintendo, either. It would present big coding challenges to developers who wish to support both Wii configurations in a single game. Those who only support the tarted-up Wii would probably suffer low sales due to small install-base when compared to the bog-standard Wii. This leads to developers not really wanting to take true advantage of the upgrade, and the most you'll get with the majority of games is an extra lick of detail, not even barely bringing it up to 360/PS3 levels. So I don't think that's realistic.
Nintendo are better of just keeping the Wii as it is, and then if they wish to get back into the running on the tech stakes - just leave that until the next console. Keeps things simple for consumers and developers. I'd say the Wii could last another couple years. If they go down the upgrade attachment route - they better be careful. They risk fucking everything up if they do it wrong.