MK 8 was hit on Wii U despite the low userbase (uninteresting console, too!).
If there's a Wii U game that can sell really well on Switch - and the console itself, btw - it's MK 8.

The game's sales were cut due to low install base, so, no wonder we are seeing this game selling really well worldwide.

I don't see it surpassing Zelda BoTW, but it might end up getting close to it.

ummm are you kidding me?? I definitely see it passing Zelda. It's Nintendo's biggest multiplayer game (SSB is close). Mario Kart as a series is a much bigger seller than Zelda. Granted this Zelda is obviously gonna be the best selling Zelda ever (should hit 10 million), and Mario Kart sales will be hurt a little by Wii U MK8 owners not double dipping on it, but I fully expect MK8D to cruise right on by 10 million sales as soon as the Switch userbase size allows it.

I don't see the sales getting hurt by that scenario because all Nintendo needs to do is announce 2 dlc packs like they did on Wii U and drop 16 more tracks in the dlcs with more characters, bikes, tires, etc.  MK will easily outpreform Zelda in the long run because the simple fact is there are more people that like Mario Kart than Zelda.  The only series that has a chance to beat Mario Kart in sales is Pokemon.

I know MK, as a series, sells a whole lot more than Zelda. But there's 2 things going against it:

It's not a new game anymore and because of that, some or a lot of the excitement from non-Wii U owners might have passed;
It's also not new enough from the 7+ million who already bought it.

I think it's biggest setback might really be that this is a 2014 game with a few new add-ons.
Not to mention that MK 9 might arrive in one or 2 years and cut into it's sales.

MK 9 won't release within a year I can guarantee that.  I'd say no sooner than 3 years.  They'll do some dlc release on deluxe I'm sure which will be enough for it to go strong for 2-3 years at least.  How many tracks do you need to be satisfied?

I wouldn't be so sure it's coming so late.
Not that it can't happen, but the majority of MK games release within a year or so after launch.

I really wouldn't be surprised if it's a 2018 holiday title.

*cough* Curl *cough*

Have people just been buying MK8D? Zelda? Both? 

Not too surprising if you look at the past Mario Kart sales. Mario Kart 7 for the 3DS broke 14 million in sales. The Wii one broke 30 million. Mario Kart 8 outsold the next highest Wii U game by 3 million and more than 50 percent. Mario Kart is huge.

It probably won't. The reason this game was ported was because Mario Kart is such a big game for Nintendo. It outsold every other Wii U game by a huge margin. Moreover, Nintendo looks to be focusing on sequels with games like Splatoon 2. I don't see a Mario Kart game selling well to change this fact. Mario Kart selling well is expected.

I guess i'm at a risk of being proven wrong about MK 8 not surpassing Zelda BoTW.

"Nintendo Switch has secured its first UK No.1 with Mario Kart 8 Deluxe.

In fact, it's the first Nintendo boxed game to get to No.1 since 2011, when the format holder topped the charts with Pokémon White on DS. It's also the first Mario game to reach the top of the charts since Mario Kart Wii in 2008."

Link: http://www.gamesindustry.biz/articles/2017-04-30-mario-kart-8-deluxe-is-nintendos-first-uk-no-1-since-2011