It's about midway through the Switch's life, so I thought it was worth comparing Switch games' sales to those on previous Nintendo hardware. And because we all know that the Switch is huge compared to other consoles besides the Wii, I thought I'd focus on handhelds.

The following table compares eleven Nintendo series by listing their best-selling iteration on each of five platforms. That means, for example, only the best-selling Pokemon game for a platform is counted, so only Pokemon Ruby/Sapphire's numbers are listed in the cell for Pokemon on the GBA. That said, I think there are some notable conclusions we can draw from this.

Also, I skipped over series that only left an impact on one or two platforms. So that's why you won't see Tetris, Nintendogs, Splatoon, etc.

Even acknowledging that the Switch's numbers will only rise, there are still some things to note.

• The Switch and DS dominate in terms of total software sales, whereas the GBA underperforms. Even acknowledging that some major Game Boy games are not included, this is still indicative of the Switch's success.
• Notable winners on Switch include 3D Mario, Mario Kart, and Legend of Zelda.
• 2D Mario is the biggest letdown on the Switch. Sure it's a port of a Wii U game, but so is Mario Kart 8 Deluxe, and that's outsold Mario Kart DS!
• Pokemon Sword/Shield is going to be the best-selling entry in the series since the Game Boy Color days, being only 300k sales behind Diamond/Pearl on the DS. Assuming the game has similar legs as X/Y on 3DS, we can expect it to sell up to 10 million more copies. That would still be behind Gold/Silver, but still very high.
• Considering how stable Animal Crossing's sales are, it's already outselling previous entries is remarkable.
• 3D Zelda outsold 2D Zelda on both the 3DS and now especially the Switch.
• Kirby is no surprise, being largely constant in sales since 2000 or so.
• DKC: Tropical Freeze on Switch sold decently enough, but is notably around the same sales as Returns 3D.
• Three Houses is roughly on par with Fire Emblem Fates in terms of sales, despite that game having mechanics that likely inflated sales
• If we get another 2D Metroid, expect it to sell poorly.

Any thoughts?

The software sales continues to impress

NSMBU on switch is selling fairly more than enough for a port. A new 2D Mario would have explosive sales if it's planned.
But what i disagree on with you is what you mentionned about a new 2D metroid. Metroid on 3DS sold poorly because it was a remake first of all on a dying system of a game that had a very good fanmade remake(AM2R). I'm assuming that if it was released on switch it would have done way better.(the same happened with starfox Zero on wii u). Sometimes Nintendo shoots itself in the foot by taking bad decisions concerning some franchises and then put the blame on bad sales. After that they say that no one wants a new 2D Metroid or a new Starfox etc...(and i still don't understand the f*ck that happened with starfox zero seriously).
In fact, i do believe that a new 2D metroid game will do well if it were planned as a sequel for fusion and with the good advertising it needs. Because we have to admit it that Nintendo had never well invested on metroid games. I hope things go well for Prime 4 and see it outsell the first prime (over 3millions sales).
That said, i see that every exclusivity on switch will outsell its predecessors considering the hardware sales including pokémons if gamefreak decides to think about quality the next time they make a game.

If SwSh has the same legs of XY it will lead to a maximum of 4-5 million in sales, not 10

And keep in mind, XY was the leggiest Pokemon since at least GS, but I do believe the DLC plus lack of a third version will do wonders for SwSh, making 23-24 million the goal for this one. Game Freak is probably releasing another main entry in 2022, so this game maybe won't experience enough time to outsell GS, I'm thinking there is a 50% chance of outselling it

I wouldn't make the 2D Mario comparison against Mario Kart. The market clearly accepted Mario Kart 8D as THE Mario Kart of the Switch. If they hadn't I'm sure we would have seen MK9 on the Switch, but since consumers accepted it (and therefore Nintendo too has accepted it) as the Switch's one and only Mario Kart and it is selling as though it is a brand new Mario Kart.

NSMBUD is not the same. It's a port of a last gen game at full price that launched in January as just a sort of post-holiday filler, and for a 2D Mario it is selling as such. It even has the Wii U branding in its name, unlike Mario Kart. I think everybody expects Nintendo to make a 2D Mario for the Switch, and it would be shocking if they didn't, which will trounce NSMBUD sales. It's basically a filler Mario game until they release a Switch 2D Mario, while MK8D is the Mario Kart for Switch. Still though, the last gen port of 2D Mario will sell over 10 million, which should suggest that a new game would likely sell over 20 million, putting in on par with the top selling portable 2D Mario's of the past.

Not sure if being "best in the series" means too much. For your last sentence for the bolded, people will mention Pokemon Sw/Sh, since it will likely set itself up to be the third or second best-selling games in the series.

As for your last point, its kinda strange. Metroid Samus Returns was well-received by critics and, outside of a few fans, I've heard mostly good things about it.

Edit: I thought about Ocarina of Time, but it was the best-selling game in the series until Twilight Princess, so my initial point in relation to your point #2 doesn't work out.

It's interesting that the GBA first party software didn't sell too hot and yet it still sold more hardware than the 3DS.  Must be that the GBA actually had an exceptionally good third party library.  I know that I personally have a lot more third party games I like on the GBA than any other handheld system.  On the other hand I don't think the GBA actually has any first party games I like other than ports.

It's actually been over 2 years since they last updated Kirby and 2 years in September since they last updated donkey Kong 

It's remarkable how stable Kirby numbers are. Also, no wonder 2D mario sales are lacking, the New Super Mario series has been losing steam since the 3DS, not to mention Mario Maker stealing its thunder.

Any chance of throwing Yoshi on there?