Xenoblade Chronicles X has been selling 4000-5000 units a week so far this year, without significant signs of dropping from that (the weakest week was January 24th). It's likely to drop somewhat over the year, but it's looking likely that it'll average in the 3000-4000 area for the non-holiday periods of 2016.
From the latest numbers, there are 35 weeks until the start of November, which means added sales of 100,000 to 140,000, pulling its physical total up to just shy of 900,000.
Somewhere around November 2016 is when the Wii U successor is likely to launch, meaning that Xenoblade X's sales will be somewhat diminished from what they would have been otherwise. It should still keep on selling somewhat, though. The original Xenoblade sold 80,000 copies the December after the Wii U launched.
Xenoblade Chronicles went on to sell 170,000 units the years after the Wii U launched (2013 and out). It's unlikely that the Wii U version will add that much, but it also doesn't need to in order to reach a million.
By physical sales alone, it's not at all unrealistic to imagine its total sales being
760,000 up to February 28th 2016
120,000 from February 28th to November 1st 2016
60,000 from November 1st 2016 to 1st of January 2017
80,000 from 1st of January 2017 and outwards.
For a total of 1.02 million.
The legs I'm suggesting here are lower than the legs of Xenoblade Chronicles, largely because the sequel doesn't seem to have quite the same cult status as the Wii version had. Both the holiday 2016 numbers and the numbers after that could very well be higher, but it depends to a large degree on what happens to the Wii U from then on.
This isn't even including digital numbers. If you add those in, 1 million is virtually guaranteed, assuming VGChartz numbers are correct.
Essentially, this means that Xenoblade Chronicles X is likely to end up having sold somewhere between 0% and 20% more than what the Wii version did. If you predicted more than that, you were being quite optimistic. So far in the Wii U's life, only one Wii U sequel to a Wii game has sold more on the Wii U - Rayman Legends (0.65 million) against Rayman Origins (0.58 million).
In short, the OP's prediction is what should be the safe hypothesis - the Wii U sequel won't outsell the Wii version. It's looking very possible that he was wrong, and that the Wii U version will in fact outsell the Wii version. It's an interesting position to start an "I was right!" thread from.