As the title says do you think Mario Kart 8 (Wii U + NS) has a chance of catching up and beating the sales of Mario Kart Wii to become the best-selling Mario Kart game of all time? Do you think the Switch version by itself can outsell Mario Kart Wii?  

Mario Kart 8 shipment figures are currently at a combined 27.45 million copies, while Mario Kart Wii shipped 37.24 million copies. Mario Kart 8 has shipped just below 75% of what Mario Kart Wii did lifetime and needs to ship another 9.79 million copies to catch up. 

Mario Kart 8 on the Wii U shipped 8.44 million copies lifetime, while the Switch version has shipped 19.01 million copies as of September 30. The Switch version has shipped 3.99 million copies in 2019 from January to September, which is down about 9% year-on-year. If this trend continues it will ship just under 3 million copies during the holiday season to bring lifetime shipment figures to over 30 million. 

Looking at the Switch version by itself it currently has shipped just over 50 percent of what Mario Kart Wii shipped lifetime.

As a side note Mario Kart DS shipped 23.60 million copies and it is inevitable at this point Mario Kart 8 Deluxe will outsell it, as it is currently 4.59 million copies behind. 

BTW yes I got this idea from @tbone51 thread "What is Zelda: Breath of the Wild Going to Actually Sell Lifetime? (25mil+ A Possibility?)," which you can check out here.

Yes...... as long as they dont release Mario Kart 9 anytime soon.

It's really looking very likely at this point. Mario Kart 9 would have to release in 2020 to prevent it and I don't think it will release next year.

Can't even rule out anymore that the Switch version alone will do it. Crazy. Just crazy.

Maybe. I hope so. The best entry should outsell the worst entry.

Yeah, I agree with that.

Combining both versions, I definitely think it'll pull it off. MK8D is shipping 1 million copies every quarter, and that's going to be way up this quarter thanks to the holidays.

Mario Kart 8 Deluxe's shipment data per quarter: Quarter (Total)
Q1 FY 2017: 3.54 million (3.54 million)
Q2 FY 2017: +0.88 million (4.42 million)
Q3 FY 2017: +2.91 million (7.33 million)
Q4 FY 2017: +1.89 million (9.22 million)
Q1 FY 2018: +1.13 million (10.35 million)
Q2 FY 2018: +1.36 million (11.71 million)
Q3 FY 2018: +3.31 million (15.02 million)
Q4 FY 2018: +1.67 million (16.69 million)
Q1 FY 2019: +1.20 million (17.89 million)
Q2 FY 2019: +1.12 million (19.01 million)

With ~10 million units to go & the legs it has, it can ship between 4-6 million units a year and can pull it off some time in 2021.

As for the Switch version, by itself. That's going to be a much tougher task. It needs to ship another ~18.3-18.5 million units to pull it off. That's going to be extremely difficult to accomplish, even for Mario Kart. I'm going to take a "wait and see" approach for that one.

When it  comes to the Switch version by itself it depends if the Switch has the legs of the Wii, with a sharp decline after a few years, or the legs more like a Nintendo handheld. Still early to tell if the Switch Lite will push sales of the Switch to levels of the Wii/DS in their peak years. 

Another factor as others have pointed out is the release of Mario Kart 9 in 2020 or if Nintendo will wait to release that until it's next console. 

Combined it looks like it will. Switch alone is possible as well, but that’s very far away. I think if will fall short, but that wouldn’t really be ‘failing’, MKWii’s legs were (are) insane.

I'll be optimistic and say that MK8D can pull it off alone in the end unless Nintendo comes with a MK9 before 2021. And unless Nintendo brings out MK9 next year already, beating MKWii by combining Wii U and Switch numbers are already in the bag.

I mean, according to PAO's post MK8D sells about 7M a year on Switch right now. I do expect the sales to slow down a bit with time, but by end of 2021 it should sit at only 2-6 Millions from MKWii (3M this holiday season plus 5-7M per year in 2020 and 2021) unless a new one came out in between, which would drastically slow down the sales of MK8D.

After this holiday the Switch version alone will be about 15 million behind. If sales remain consistent or have small drops it will surpass Mario Kart Wii sometime in 2022. Is it possible, sure. It is garaunteed, no. Depends on when MK9 releases and the legs of the Switch beyond 2020.