Question is not if Mario Kart 8 Wii U + Switch will outsell Mario Kart Wii, question is if the Switch version alone will be able to do it... which, believe or not, is looking very likely right now.

Have some charts.

After 10 quarters, Mario Kart 8 Deluxe has shipped 19.01 million to Mario Kart Wii's 24.01 million at the same point in its life. Despite Wii's lead, 8D has sold more in each of the last five quarters, shrinking the gap from a peak of 7.04 million at the end of Q5 to just 5.00 million at the end of Q10.

The attach rate is also an interesting comparison. Both titles launched at the same time of the year, but their early attach rates diverged wildly because of different launch timing in their respective consoles' life-cycles. Wii's attach rate never changed drastically after the first few quarters, and it continued to grow at a slow, steady pace for years afterward. Now, it stands at 36.64%.

In contrast, 8D has experienced more fluctuation in attach rate over the same period, making it difficult to estimate where it will go in the future. Over the past year, the rate has declined by about 5.3 percentage points. 8D still has a considerable lead in this category, and the fact that Switch will sell more than Wii certainly helps. However, MK8D will need to maintain this buffer if it hopes to surpass MKWii because of the threat that Mario Kart 9 poses to its legs.

I hope we get MK9 fairly soon. In that case, MK8 probably will not outsell Wii, because 9 would be on Switch and take the wind out from under 8D. But, considering that 8 is still selling well, and that they've released it twice (so Ninty probably figures that have to make a truly completely new game for the next one), I don't expect 9 until at least 2021.

I reckon combined mk8 could outsell wii, but the switch version alone, would be a long task, if mk9 doesn’t come out any time soon, then it could be a possibility. Impressive legs nonetheless

It all depends when Mario Kart 9 Comes Out. Let's say if it never releases on Switch, I believe MK8DX alone will outsell Mario Kart Wii Globally.

MK8 Deluxe is already outpacing the MKWii in sales within the same time period. And the Switch's install base has much more dedicated gamers than what the Wii had, so there are more potential buyers for MK8DX for the Switch, which is why I believe MK8DX will outsell MKWii.

But we have to see when MK9 comes out, if it gets released 2020 or 2021, I dont think it will outsell MKWii. But I could see Nintendo sitting on MK8DX for a while by how amazing the game is selling. I'm predicting towards the end of the Switch's life we'll see MK9 around 2023

There is a good chance it will do it combined, Switch alone can still do it but it needs longer

A lot of people keep mentioning Mario Kart 9, but I am not convinced Nintendo will release Mario Kart 9 on the Switch.  I mean, why would they need to?  Just look how Mario Kart 8D is selling.  Why release a new game that is just going to eat into the sales of the old game that is selling like no tomorrow?

Mario Kart 8D is a lot like GTA 5.  Technically GTA 5 was a PS3 game, but it also is the best selling game on the PS4.  Why would they release GTA 6?  (So far they haven't.)  Since GTA 5 keeps selling there is no reason to release GTA 6.  By the same token, since Mario Kart 8D keeps selling, there is no reason to release Mario Kart 9.  They could just keep developing it, and then release it as a launch title for the Switch's successor.

With WiiU combined, it should outsell MKWii by holiday 2020. Without, sometime in 2022, that's if MK9 doesn't happen before that time. So Switch itself has a potential to make it happen, but realistically, I doubt we're going to wait that long for the next MK game.

I dunno mkwii's legs are insane. In places like SE Asia and Korea where Wii U wasn't introduced they kept selling Wii mini+mkwii bundles well into 2015. That's what NINTENDO OF KOREA did anyway. And apparently MKwii is STILL tracking in the current quarterly report lol.

MK8D is outselling MKWii each quarter right now. While it will get a big boost mid next year (bundle or what was the reason?), sales totally falter after that. So I'm pretty sure that MK8D will catch up a lot during that period - unless a MK Switch comes out in between, and even then it could still achieve it over time.