I state that Mario Kart 8 Deluxe should easily reach 20 million copies with this rhythm, and maybe go much further.
The only Nintendo franchises that have the potential to beat this game sales are historically two.

The first is Pokémon. But not just any Pokémon, it must be a "wow" video game like Breath of the Wild or Super Mario Odyssey.
The second is Super Mario, in particular the 2d titles like New Super Mario Bros. Not porting or remastered, but a new unreleased game.

Of course also a Mario Kart successor could beat MK8D, but it's not 100% sure that a new one will come out on Switch.
It depends on what kind of business they want to push forward, the team dedicated to racing could also be put to make new IPs or other old and forgotten series, as far as we know.

For now it's more likely Mario Kart 8 Deluxe will remain the best-selling videogame on Switch in the future, because it's clear it's purchased by a good percentage of new users as essential.

Yeah same for me, even more if RetroStudio is working on it. The rumours of it having a hub and an "adventure mode" sounds amazing to me.

I'm 100% sure Smash Bros already surpassed 10m, so it's a matter of time until it reaches 11m. Pokemon 2019 will surpass all of them though.

Well it's not cause people thought it was going to be presented at E3 and didn't happen that it really isn't in the work.. i don't think the rumor was talking about an E3 reveal.

I don't think there will be a Mario Kart 9 on Switch unless Nintendo does multiple generations of the Switch console in a similar fashion to iOS. I also don't think any other foreseeable game will surpass Mario Kart 8 Deluxe, at least not permanently. This is the other game that is primarily driving sales, people see big local multiplayer matches of Mario Kart 8 Deluxe during lunch hours at the office (and I assume at schools too), and want to take part.

Personally I think that Smash has a shot getting passed MK8D in the midterm but would face the same fate as SMO by getting out legged by MK8D. Excatly the same could happen with Pokemon 2019.

Am I the only one that could never get into any of the Smash games? I discovered the series on the Gamecube, but wasn't my cup of tea. People who think Smash will dethrone Mario Kart 8 Deluxe believe it will do so because they believe it's the "bigger" franchise, or because they say Mario Kart 8 Deluxe is just a port? Personally, I don't see the appeal to Smash but I guess I'm in the minority. Mario Kart is the ultimate party game for me. It's the only game everyone can pick up and play at social gatherings, that doesn't require reading, let's everyone have their turn in a reasonable amount of time, and that is just immensely fun.

It has to do with how long Nintendo is expecting the life span of the Switch. If the successor should release at 2023 or ealier, I won't expect a new 3D Zelda, Mario Kart or Splatoon on the Switch. I estimate that Nintendo would make sure to have a great first year for their new system as they did with the Switch. Therefore they would save those games for the first year of that new console.

But I do see sequels for Mario Party and Super Mario Odyssey arriving on the Switch as well as the different Pokemon games that you mentioned.

I tried twice to get into Smash. The first time was at my cousins house with Super Smash Bros. Melee. Sure he beat the crap out of me but I felt no appeal playing that game.

Few years later I borrowed from a friend Super Smash Bros. Brawl. I gave it around four hours but I got so bored playing that game. Since then I never touched any game of the series, but I am still happy for those who enjoy it. 

Mario Kart is a force to be reckoned with. I tried the Game comparison tool on this website and compared Deluxe with MK7 and MKDS and guess what... The game is even outpacing the DS iteration. So it might end up at 30 million, becoming the 2nd best selling Mario Kart of all time!