I think Deluxe will sell well in the beginning, but drop faster than normally for a Mario Kart. Still, if the Switch continues to sell well, I think it can hit 8-10 Millions

Just we having remaster of most popular Wii U game, system that sold only 13-14m, lotsa Switch owners (curent or future ) didnt had Wii U, and for them MK8D is great new game, and actually lots a Wii U owners will do double-dipping maybe because of full handheld play or new Battle Mode. We dont know if Switch will have MK9 at all, or if it will, when could be arive, so people who want MK will buy MK8D, they will not wait for MK9 when they dont know for sure if will arive on Switch at all or when exatly could arive.

5m is very positive, but not impossible, don't forget MK8 on Wii U has attach rate around 60%, you can bet that lotsa people thought that was impossible also. MK8D is in any case easily 5m+ seller on Switch.

Because only a few actually have a Switch already and since there ain't too many good games out on the system yet, sales will probably be initially frontloaded and then drop sharply, setting on a sales level which it will keep for many weeks and months after that. But that level should be relatively high due to the high influx of new Switch owners looking for some great game to play until much more big hitters are available for the console.

Alson Wii U MK8 owners will most probably buy the game either early on when they got their Switch, or not at all since they have already the Wii U version, with little in between.

We don't know if or when MK9 will release so it could go either way. They could expand deluxe with even more tracks and release MK9 in 2019 or 2020. Or they could release MK9 in 2018. Too many variables to make a prediction.

Boo. Opening Pandora's Box with the question but low-bailing your prediction to play it extra safe. That's as entertaining as crowning a Pawn Queen when you already have two Rooks and the enemy's King hasn't got any piece left.

Being the most popular WiiU game makes it for a certainly profitable avenue to remaster and port, but that doesn't strictly has to indicate that it will sell the same amount (let alone 12 million, like OP asks). I'm not really sure the power pull of Battle Mode for most people that bought in on WiiU, but I assume it won't be enough to consider that many amount of double-dips (outside people just wanting to pay for the same game again but in a different platform). Some people that missed it on WiiU can also consider, but I'm not banking either on that many people wanting to play a remaster, and they might entertain themselves with everything else Switch has up this point until the inevitable (because it's almost a given that will happen, and people know it) Mario Kart 9.

5 million is overly positive for the first year. That would mean doing over the 65% of the sales that the game did on the other console lifetime in just a span of six-twelve months, and I just don't see that happening.

Both in fact. Which I explained before but apparently not good enough

The level it will sell over the months starting May will be higher than usual, but it will drop faster down to that level than the previous itinerations of Mario Kart, where the sales stayed relatively high over months before settling on it's respective level

I think a big factor here for sales will be what percent of Switch buyers never had a Wii U?  If we be very conservative and estimate the dedicated gaming hardware market to be 100 million, that would mean over 80 percent of possible Switch customers never bought a Wii U.  Which means we could see a more pronounced version of the Last of Us Remastered effect, where peopld who never owned the previous system buy the new one in large numbers and thus drive the sales of a remastered exclusive higher than one would initially think.

No, I'm just saying it will sell huge numbers at first, then settle down to it's (high) cruise speed after that. Which all Mario Kart more or less do, but the transition between the two is normally several months long. I don't think the transition of MK8DX will be several months long, just one or maximum 2 months.

I would pay $60 for the new battlemode only, I think... let alone the rest of the game lol

It will surpass 10 million easily.

http://www.vgchartz.com/gamedb/?name=mario+kart