Nope. It doesn't stand a chance. It'll be between 5mil-6mil lifetime maybe slightly higher but not close to 7mil

I think this is the highest attach rate any title has at the moment. It's likely that it will go down a little by the end of the Wii U's life, but it should still stay over 50% attach rate. That's quite massive. Mario Kart Wii had an attach rate of 35%, Double Dash 32% and Mario Kart 64 of 30%. Super Mario Kart had a significantly lower attach rate, but that's largely because that's before Mario Kart was Nintendo's arguably biggest franchise. Super Mario World, back then, had the highest attach rate of roughly 42%

We're seeing the same trend on the 3DS, with MK7 at 21% attach rate, while MKDS had 15% and the GBA one at 6.7%.

Partially, it's down to Nintendo's games having growing attachment rates all around, but it's more than that down to Mario Kart having become the heart of what Nintendo now is.

While Nintendo of the past (NES-N64) was arguably the greatest developer at creating challenging and large-scale games, they're not that anymore. Or at least, they don't hold the title with the same power, due to there being so many other excellent hard or large games. Today's Nintendo is rather the best at having games that are fun together, with 7 of the Wii U's top 10 sellers being games that are far more fun when there's somebody else in the same room (MK8, NSMBU, Smash, Nintendo Land, 3D World, NSMLU, Wii Party U, 2 titles being online games (Splatoon, Super Mario Maker), and only one large scale, single-player game (Wind Waker).

So in a sense, it shouldn't be surprising that MK8 is doing as well as it is, because if you're buying a Wii U, then you're very likely to be interested in games that are about having fun together. And if you're interested in that, virtually nothing tops Mario Kart.

I think the reason the attach rates are higher now is because there's less choice on their consoles.

SNES had great games from tons of third parties, Wii U just has Nintendo games so of course most are gonna have MK.

it will become the 4th best selling Mario Kart ever

Well in that case every single Nintendo game would have great attach rate on Wii U!?

MK8 has such a great attach rate because it's great game with mass appealing.

A bit sad for Double Dash, as it's still my favourite Kart, but happy for MK8 - Nintendo put so much effort into the game, would hate to see it go unappreciated. Judging by quality it should be ahead of MK Wii already.

This is probably correct for Mario Kart, but I wonder if we could further refine their system's appeal more than that by using other titles. For example, Smash Bros is arguably targetting that same niche, but it sold much better on the 3DS. The Mario platformers are doing well, and have local multiplayer, but at the end of the day it's hard to argue that multiplayer is the focus of those games instead of just a feature. The only other big hitter is Splatoon, which is the opposite of a Fun Together game.

I suppose what I'm getting at is that if strong local multiplayer is the system's intended focus, their heavy hitters seem to be a bit muddled in following that goal.

I don't recall the 3DS getting a lot of 3rd party support.

Yet the likes of Square Enix, Capcom, Level 5, Ubisoft, Renegade Kid, Namco, Konami, SEGA, Marvelous AQ, Tecmo, Atlus, Activision and so on all have games on the 3DS and the first party titles have sold more which debunks your earlier statement.

In Japan, 3DS has the best 3rd party support. Which is why people dodge comparing PS4 to it lol.