A million? Really? That seems unlikely. 500k a stretch but posssible a million would kind of blow my mind.
Why is it so hard to imagine? This is not just THE biggest Wii U title to launch thus far, it's also one of THE biggest games on any system in this new generation to launch, period. It's going to be one of the single biggest releases of 2014, and there is major hype surrounding it. There are tons of pre-orders, but more importantly, LOTS of people are going to buy it. Think about it for a moment. Of the 6+ million Wii U owners right now, you have to believe most of them are pretty hardcore Nintendo fans, aka already major Mario Kart fans. Just out of the current existing install-base, it's actually very easy to believe that at LEAST 1 million of them, myself included, is going to run out and buy MK8 on launch day.
Then of course, there are all the people who have been on the fence/waiting to get a Wii U until something like Mario Kart or Smash Bros. came along. Well, it's here, and many of those people are now going to be ready to make the leap. Especially with a bundle available on launch day in many regions.
So you combine all that? I'm not saying it's 100% for sure that MK8 will do a million Week 1. But I'm saying it's absolutely very possible.