One mario kart game?? Mario Kart outsold Zelda on EVERY Nintendo platform by a huge margin. Snes, N64, GCN, Wii. 

Based on VGC data over the last few generations here is a ratio of MK sales : Zelda sales (top selling Zelda of the generation)

As MK Wii is an outlier I believe the averages excluding Wii are better to use as an indicator.

MK U is currently at nearly 4.5 million, but it is continuing to sell well so it will probably get to 6 million at least.

We can see a clear pattern. MK always sells better than Zelda, and the ratio (excluding Wii) ranges from 1.3:1 to 1.9:1 with an average sales ratio of 1.6:1.

If we assume WIi U approximates to GC as they are likely to end up with similar install bases then we can estimate that MK U will get to about 7 million. If we use the average MK:Zelda ratio this indicates Zelda U should sell around 4.4M. If it gets a similar sales ratio to GC then we should expect Zelda U to sell around 4.65M.

Again assuming MK U gets to 7 million, for Zelda to crack 5 million the ratio will need to be 1.4:1. This is within the historical Zelda:MK range. So 5M is very possible assuming MK gets to 7M. If MK get to 8M then the ratio would only need to be the MK:Zelda average for Zelda to get to 5M. If MK U only reaches 6M then the ratio would need to be 1.2:1 which is a ratio Zelda has never achieved.

Zelda is a very stable franchise which has not seen a great deal of growth over the generations, with OOT being really the only game that significantly stood above the rest.

I think Zelda U 5 million+ is possible, but unlikely.

No not by a huge margin everytime and thats not the point^

Zelda has never been as big a seller as people seem to imply. The only really big seller was Ocarina of Time. Then Twilight Princess did pretty well but largely because it got 2 releases and was a launch game with the Wii.
It's never really come close to Mario Kart or something like that. So I seriously doubt 5 million. I think maybe 2 million around launch and then up to something like 3.5 over its lifetime.

MK 8 is selling at a steady pace for 8m+ lifetime. But even without using the zelda - mario kart ratio, the hype over zelda U seems enough to think it will easily sell 5m+.

I dunno. Obviously you frequent websites and forums related to Nintendo much more than I do. But not being a member of the Nintendo inner circle of fans I am not perceiving  a huge amount of hype. So the hype possibly is only within the Nintendo fanbase, which might not be enough to get it to 5 million.

Zelda U will probably need to be received almost as well as OOT to hit 5 million on the sort of install base Wii U is looking to achieve. And of course Zelda U could achieve that sort of acclaim. The Wii U is a suffiently powerful machine that Nintendo can do things with Zelda that it has never done before, and I'm sure they will make excellent use of the gamepad. It's still an up hill battle to 5 million though.

My metric ... take whatever some Nintendo fans predict for sales and chop a generous 30%-40% or so off that number to get what will happen in reality.

There's always gross overinflation of sales expectations of Nintendo games here I think even VGC gets caught up in it, that's why the Wii U and 3DS are often times overtracked big time.

It's coming fairly late in the Wii U life cycle too, that will limit its sales potential. Zelda: WW (GCN), Zelda: TP (Wii), Zelda: OoT (N64), Zelda: LttP (SNES) were already out in the launch alligned time frame for their platforms by now. That art style is not going to help it sell either, sorry, but the more cartoony styled Zelda games just don't sell and particularily in the modern marketplace ... good luck with that (not saying I approve of that POV, but that's just the reality of the market). 

Even MS had big problems selling something like Sunset Overdrive, which is a third person shooter that had a fairly large marketing campaign behind it, I think the more cartoony art style hurt it. Mario Kart can get away with it because its Mario, but if you're making a "serious" action-adventure game, it's a detriment. 

"Zelda U will probably need to be received almost as well as OOT to hit 5 million on the sort of install base Wii U is looking to achieve. And of course Zelda U could achieve that sort of acclaim."

What would make you believe that a franchise in decline will have a hit game that is recieved as well as the game that is widely considered the greatest of all time even today?

What do u mean by the franchise is in decline?

OoT is the most overrated game of all time.