MK8 has proven to be a beast, being the best selling game on both Wii U & Switch, by a long shot. The Switch version is on the cusp of outselling MKWii and sales have generally increased with each FY (likely helped by it being the go-to holiday bundle for Nintendo).

Do you think both versions of MK8 combined can outsell Wii Sports (and become the best selling Nintendo game)?

I'm combining both versions because I feel like the Switch version by itself won't do it, but it has a chance when combining the Wii U version. It's not really unfair to Wii Sports either, especially since Wii Sports was bundled with most Wiis outside Japan (if anything, bundling is a bigger advantage).

Ultimately, I feel like it could very barely happen. I feel like MK8 on Switch will sell 75 million and combined with MK8 Wii U's 8.45 million, we get 83.45 million, barely ahead of Wii Sports' 82.90 million.

As of June 30, 2021, MK8 DX has sold 37.08 million. As of March 31, 2021, MK 8 Wii U has sold 8.45 million (let's face it, that number likely ain't changing). Total sales for the two are 45.53 million. They need 37.37 more in order to match Wii Sports (essentially, MK8 DX needs to more than double its current sales). I feel like Nintendo could bundle it more often with the Switch as we get closer to the end of Switch's life. Also, I feel like 2022 is the year where we will get a little bit of DLC for MK 8 DX (nothing major...just another row of vets/newcomers and 2-4 cups of retro/new cups), considering it is the 30th anniversary of MK. More bundling and more DLC would obviously spur sales.

What do you think MK8's combined LT sales will be at the end and can it overtake Wii Sports?

It already outsold Wii Sports in 2017, most or Wii Sports copies were bundled

I don't think MK8DX will reach 70M, but I sure do hope it will.
80M is the absolute ceiling for both versions combined imo.
I think this year is the last year in which it will outperform the last.

MK8DX sales as of December 31st 2020: 33.41M
MK9 could be a launch title for the Switch successor, which I expect in 2024.
Yearly sales projection:
2021 - 10M
2022 - 9M
2023 - 7M
2024 - 5M
2025 - 2M
Endresult: 66.41M
With MK8: 74.86M
So around 75M is what I expect, which is an insane number.

Very highly unlikely.

No but the Switch version alone is definitely crossing 50m, and combined both versions should cross 60m. Putting it just 10m or so behind a literal pack in for one of the most successful consoles ever. No one expected this game to be as massive as it js.

Honestly this is the first time that I’ve considered that the two Mario Kart 8 games could even outsell the original releases of Super Mario Bros.
I reckon the two games combined will definitely sell 59 million and 65 million is definitely possible if Nintendo doesn’t announce a sequel for Switch in the next year or so, but I can’t see Switch having the legs to get it to 70 million with Wii U and Switch sales.

What if I told you that it wasn’t the game that was given away for free, but the console?

It wasn’t free, it was an integral part of the Wii bundle. When people were first buying the Wii, Wii Sports was close to the main reason if not the main reason itself.

In my mind it already has. I don't count the majority of Wii sports sales as sales.

No. MK8D will sell over 50 million, maaaybe 60 million. So the two MK8's combined will definitely sell over 60 million and might even finish close to 70 million combined.
82.90 million is out of the question.

It's at 37 million now.
Let's say 41m by end of year.
47m end of 2022.
52m end of 2023.
56m end of 2024 (and Switch 2 comes out at that point)
Maybe it works its way up to 60 million eventually, so the two versions combined top out a bit under 70 million, well off Wii Sports 82.90 million.