You have to go back nearly two decades to find the last time Smash came close to Mario Kart, and back to the DS for the last time a 2D Mario beat it.

With Switch anything seems possible in this regard, just compare SMO and BotW to the other 3D Mario/Legend of Zelda games

i rebought it because of the battle mode. and i firmly believe this will be the switches mario kart

Update April 21st 2018 :

4 new games in the list : Nintendo Labo VK (entering at #36), Nintendo Labo RK (entering at #61), Wild Guns Reloaded (entering at #92), Neo Atlas 1469 (entering at #118).

Weekly Top sellers were : 1) Nintendo Labo VK ; 2) Mario Kart 8 ; 3) Kirby!!

Number of 100k sellers : 49 (+1)
Number of 200k sellers : 35
Number of 500k sellers : 18
Number of 1M sellers : 10
Number of 2M sellers : 4
Number of 5M sellers : 4

Update April 28th 2018 :

7 new games in the list : South Park - TFBW (entering at #91), SD Gundam GGG (entering at #104), Naruto Shippuden - UNST (entering at #112), Max - TCoB (entering at #120), Shantae - HGH (entering at #121), Darkest Dungeon (entering at #123), Cube Creator X (entering at #126).

Weekly Top sellers were : 1) Nintendo Labo VK ; 2) Mario Kart 8 ; 3) Splatoon 2

Number of 100k sellers : 50 (+1)
Number of 200k sellers : 36 (+1)
Number of 500k sellers : 18
Number of 1M sellers : 10
Number of 2M sellers : 4
Number of 5M sellers : 4

Really sad that South park, Shantae and Darkest Dungeon didn't get more, as they merit much more than that.

i think that South Park suffered a lot from the major delay between switch launch and the original launch, then suffered a bit more because they offered stick of truth free with the purchase of fractured but whole on other systems, and not on switch

Yeah, and the bugs didn't help matters, either.

However, no excuse for the 2 other games.

Well, Shantae was released over a year ago. Most people who wanted the game already got it digitally.

Kart 9, which is probably a 2020 or 2021 title, will almost certainly crush everything on Switch that came before.  The install base will be much larger, and Kart is always a sales beast.