Mario Odyssey is a big game. No doubt. However you are severely underestimating how good a 2D Mario game can do. We are talking about 30+ million sales series here. NSMBWii was incredibly popular back in the day. However I would consider its performance handicapped. Thing is that Nintendo has not been treating 2D mario games with the respect it deserves. Here is an analogy using the Zelda series. 

Imagine that instead of Breath of the Wild we got Skyward Sword Switch. Skyward Sword Switch will have the same worlds,music,story and characters as Skyward Sword on the Wii. The only difference is better graphics and level design. Now imagine if  Nintendo released Skyward Sword 3DS just 3 months earlier. The main gimmick of Skyward Sword 3DS is that you must collect a bunch of rubies to get a secret ending of sorts. However Nintendo says its a different game since the level design is different and all. However it uses the same worlds,music,story and characters. I don't know about you but I would be pretty mad about this if I was a Zelda fan. This is how 2D Mario fans feel about the series. NSBWii is lucky in that it didn't feel the fatigue yet. Despite that the game feels stale and unambitious because in a lot of ways it just feels like the DS game but blown up into a Wii game. 

A proper 2D Mario game with the love and care as say Super Mario bros 3 or even Galaxy will blow Mario Odyssey out of the water. A proper Zelda like Breath of the Wild blew up the sales. I expect them same thing to happen to 2D Mario if given the same respect. 

mario kart on wii did 37 million, i think its safe to say mario kart 8 deluxe which blew away mario kart wii in quality will not touch those sales, probably ever. it's always better to look at the latest entries  of franchise to judge sales, not a outlier.

Only if you are gonna ignore install bases. In the basis of this premise, if there are any outliers, they will be the wii u versions. MK8D won't sell 37m mainly because it is a port. However, i think it's safe to say that once an all-new MK9 invariably hits the Switch then it will have a shot at similar numbers if the Switch userbase can grow to a similar size as the Wii.

mario kart 3ds  has a install base of 70 million mario kart ds 155 million, but like i said its best to look at the latest entries, like GT and halo series are on a huge decline, 2d mario hasn't had 20 million selling game in over 10 years.

You really don't have a clue do you?

what are talking about? we have 3 releases of 2d mario in the past 10 years, mario maker, NSMB 3DS, and NSMB wiiu, none have even topped 15 million.

Kind of hard when the console two of those games were on didn't even sell 15M units don't you think?

mario maker is also on 3ds.

The bar is pretty low in EU though. Wii sold to 6% of Germans, 10% of Frenchies and 14% of Brits. Switch is as popular in UK as it is in Germany (probably more) when population is taken into account.

what are talking about? we have 3 releases of 2d mario in the past 10 years, mario maker, NSMB 3DS, and NSMB wiiu, none have even topped 15 million.