There are plenty of reasons why Mario Kart 9 makes sense on Switch:

1. Switch has not received its own Mario Kart yet and a prolonged lifespan for the system is easier to accomplish when you put system sellers on it in the second half of the lifecycle.

2. A new Mario Kart will drive profits for years, especially with the inevitable DLC.

3. The game can be easily repurposed for the Switch successor as a Deluxe version that includes all previously released DLC and some extra goodies. Most people don't buy DLCs, so Nintendo can put together an attractive package (read: system seller) even if the Switch successor is backwards compatible.

The reasoning that is usually put forward against Mario Kart 9 on Switch is pretty much the same as it is for a new 2D Mario game. "Oh, NSMBU Deluxe is selling so well, there's no need to make a new game for Switch."

It would be foolish of Nintendo to make the second half of Switch's life all about new IPs. They need proven IPs as tentpole games each year too. They are making Breath of the Wild 2, there will definitely be another Pokémon gen, another new 3D Mario game is likely too. I don't see why some IPs should be getting multiple new games on Switch while Mario Kart and SMB don't even get one, especially because Mario Kart and SMB are bigger IPs than the aforementioned.

There's no need to save up Mario Kart 9 for the Switch successor when the new console will receive new games anyway. Mario Kart 9 is bound to be a 30m+ seller on Switch and the hardware interest it creates will benefit all other software too.

If Nintendo decided against Mario Kart 9 on Switch, then we'd have a span of 10+ years without a new installment in Nintendo's biggest IP. It's already bad enough that Nintendo did this very thing to SMB (their biggest IP at the time, no new game for 16 years; Super Mario World in 1990, New Super Mario Bros. in 2006) and is about to do it again (New Super Mario Bros. U in 2012), so adding Mario Kart to this very shameful treatment of popular IPs belongs on the list of the dumbest things Nintendo could do over the course of the next several years.

1 and 2. I'm sure Nintendo would have released 9 on Switch it 8 deluxe had sold normally for a port, but that's not the case. The game still sells like crazy, why waste a game on a system that cuts the legs off of another? Not to mention having MK9 for Switch 2 launch will help that console sell like crazy, so why cut the legs off that console as well before it even stands?

3. Well sure, if they were going to release 9 I hope they follow your idea here and re release it in better form on Switch 2. The problem with this is many won't like the precedent they are following. We already hear an endless line of complaints from people annoyed by them porting Wii U games to Switch, so if they continue that trend of making Switch games port to Switch 2 those same triggered people will continue in their ways. Not really a problem for Nintendo as they will sell more games that way. If, however, 9 is exclusive to Switch and 10 doesn't come out many years into Switch 2's life because of that, then this is not a good idea.

Also, that 10 year gap would be for just this gen. These Wii U re-releases are happening because Wii U bombed and Nintendo is trying to salvage as much profit from good games that didn't sell to their potential in the previous system. Switch is a success, and so are it's games, so we won't have that problem with Switch 2 and should enjoy a healthy amount of original exclusives rather than Deluxe ports. I expect MK9 on their next device, MK10 on the one after that, and so on (unless they have a device bomb and then they may do the deluxe MK treatment again).

Lastly, there are a number of unexplored ips this gen that have done well before, some even recently, that they haven't tapped yet. Kid Icarus, Mother/Earthbound, Advanced Wars, Mario and Luigi, etc. I would rather they focus on making their lineup varied, appealing to a wide range of gamers, than just focus on sequels to already successful Switch games, but that's just my opinion.

Mario Kart 8 DX is such a huge title. People that are worried about its legs being cut by MK9 can rest easy. Most of the switch owners will be purchasing both games, I can assure you.

I don't understand this perspective. If you have bought 8D, and 9 comes out on the same system, you are far less likely to buy it than you would be if it came out on a new system. It's not like these are games with new stories people want to explore. Millions of sales for both game will poof, be gone, because you released two games on the same system.

If you don't own MK on Switch yet, and 9 is out, you now won't buy 8D, whereas maybe you would have bought 8D on Switch AND 9 on Switch 2.

Nintendo will do whatever leads to the most sales and least cannibalism of sales overall.

Now, releasing a new Diddy Kong Racing Game, or F-Zero, will lead to no cannibalism of MK, more thirst for MK9 and Switch 2, and success for other ips all the while appealing to fans of those series that felt neglected.

It's obvious why you can't see it. You are only putting this from your own perspective or the gamer that frequently visits gaming forums perspective, that represents a really tiny group.

Most of people don't wanna buy a new hardware to get a sequel of the game that they enjoyed playing on the system that they currently own. 

I know what I am saying bc I worked at a game shop during the MK8 release on 2014 and I was frequently asked if there's a wii version for MK8 after I explained that wii u games can't run on a wii system.

It's cool and all having other racing games from forgotten IPs, but Mario Kart is the brand that gets the attention. Having 2 MK on the same system would be mind blowing for many. Too hard to miss.

There would also be many people who would absolutely buy the game on the next system if Nintendo had released it exclusively for that and utilizing it's power that won't buy it on Switch 1 because of redundancies. So then the question becomes which one gets them more money in the long run:

A) Release a second Kart on Switch. Get some portion of the overall MK8 crowd to buy it, some new people altogether to buy it, but then sabotage MK8D sales going forward due to cannibalism and make the demand for MK10 fall down for years because many MK fans will have already been recently satisfied.

B) Release 9 on their next device. Lose out on a couple years of potential Switch 1 customers by making it a Switch 2 exclusive (though the final two years of a console aren't as lucrative as the first 4-5 anyway as sales always fall off). But then MK8D will keep selling (at no extra work for Nintendo) and the demand for 9 on Switch 2 will be so high that it sells like crazy when released.

"MK8 is easily the best kart game to date". With such bad item balance, hell no.

"it will take time to develop something better."

How much time do you believe they need? This is the seventh year since Mario Kart 8 first released. Are you people even counting? They should have enough time to release a new Mario Kart this year, and then still have another one cooking to be released for the next Switch by year 3 or even year 2, as the Switch still has crazy momentum and won't need replacing any time soon. Mario Kart 9 on switch is the most logical course of action to ensure that momentum keeps rolling.

They haven't released a single new DLC pack or track for the game since early 2015. I played hundreds of hours on WiiU, and even though I always plan to buy it again for Switch, part of the reason I put it off is I still have mental fatigue from the time I sunk in. Even Switch only owners will be reaching that point sooner than later.

Why would you want to blue-ball that huge player base while missing out on massive guaranteed sales, along with a big late-life boost for the Switch, just to put it off several more years for a system we know nothing about?

Because MK9 will have massive hype on Switch 2 launch leading to crazy sales, which will then lead to better sales for all their games. 3DS bombed hard in its first 7ish months on the market until a price cut and MK7 came out. Wii U bombed in part because MK8 didn't come out until a couple years after Wii U released. Switch sold like crazy because it had MK8D right at launch window.

If they release 9 now, I firmly believe 10 won't be ready until 1-3 years after Switch 2 comes out, which would not only sabatage their new system's hardware sales but also all the other game's sales. Mario kart is the bandwagon a lot other games thrive off of ever since the Wii game. I don't think it's worth it to them to release 9 now (which would sell great, but will diminish sales of 8D significantly), and lose all the money from a mediocre Switch 2 launch without a game.

And I don't think releasing 9 on Switch and Switch 2 will work like it did with 8/8Delux because so many more people have bought a Switch and will be familiar with 9 on it that they won't be as interested in getting it on Switch 2, further hurting sales. Nintendo is focused on the long te profits across all their games and hardware and not focused on the profits that one game could produce right now.

Mario Kart 9 at launch with a Switch 2.... that would be the smart move.