Let's put it this way. Nintendo's own financial data indicates that first-party software sales are in any given quarter consistently between 70% to 90% of all software sales. That by itself should indicate how dependent Nintendo is on their own first-party output. While exclusives are still important for any console brand, they are absolutely the lifeblood of Nintendo consoles.

Also, let's look at it from a different angle. The list of best-selling games on the Switch is dominated by Nintendo titles. Here's every multi-million seller to date:

Out of 52 games on this list, only seven were neither developed by Nintendo nor were based on a Nintendo IP. The remaining 45 games on that list amount to 528.05M copies, or 46.6% of the Switch's total software sales. And that's just 45 games! There are a lot more Nintendo games on the Switch than what's on this list.

No matter how you look at it, first-party titles represent an absolutely massive share for software sales on Nintendo systems, and they utterly dominate the best-sellers list on Nintendo systems as well. This is well beyond what you see with first-party titles on PlayStation or Xbox.

SOURCES

https://www.nintendo.co.jp/ir/pdf/2019/190131_2e.pdf
https://www.nintendo.co.jp/ir/pdf/2020/200507_4e.pdf
https://www.tweaktown.com/news/91457/nintendo-made-27-billion-from-first-party-games-across-switchs-lifespan/index.html
https://en.wikipedia.org/wiki/List_of_best-selling_Nintendo_Switch_video_games

There are two realities that need to be acknowledged whenever this topic is brought up.

1.  Nintendo is and essentially always has been the top software publisher in the video game industry.  This was true starting in the mid-80's, and it's still true today.  

2.  In spite of #1, Nintendo hardware doesn't sell well unless it has a good quantity of third party games.  These don't have to be AAA games, but there has to be other games there to fill in the gaps between major releases.  Usually Nintendo releases 1-3 major games every year.  If those are the only games on the system, then it feels like there are software droughts where the Nintendo system owner feels like there is nothing to play. 

Many Nintendo games have broad appeal like Mario Kart, Zelda or Pokemon.  That's why they sell a lot.  People play the other games, but they tend to appeal to more specific niches.  Some people really like Octopath Traveler and others really like Stardew Valley (for example), and there are tons of other high quality games that appeal to more niche tastes.  These games really help fill out the game library of a Nintendo system even if they aren't mega sellers like Mario Kart.

What people really want when they buy a Nintendo system is major first party releases supported by a large quantity of third party games.  This is what made the NES successful and this strategy has been present on every Nintendo system that has sold at least as well as the SNES.  Yeah, major Nintendo games sell a lot, because the games are that high quality and popular.  Third party games could sell well on a Nintendo system too if they were high quality and popular.  Monster Hunter Rise, Street Fighter II, Just Dance and most Dragon Quest games have sold really well on Nintendo systems.  However, most third party games end up filling a niche roll and are still successful relative to their budget, but they don't usually compare to major Nintendo releases.

I'm not surprised third party sells poorly on Nintendo consoles. Third party is in the business of making money, there was always a logical reason why the Switch had poor 3rd party support.

I'm not convinced 3rd party for the switch 2 will be any better.  Developers go where the money is.  

Witcher 3 on the switch took a lot of effort and didn't even sell a million.  

Not gonna lie, I would be really interested in how 3rd party games typically sell on PS and Nintendo systems. Not in numbers, but what their pattern is.
In my head, I have this image, where (lets take JRPGs for example) games sell better on Nintendo-systems initially, but slow down faster and maybe even earlier
resulting in a sales-curve that looks somewhat like this:

Does anybody have any data about this?

I'm not surprised third party sells poorly on Nintendo consoles. Third party is in the business of making money, there was always a logical reason why the Switch had poor 3rd party support.

I'm not convinced 3rd party for the switch 2 will be any better.  Developers go where the money is.  

Witcher 3 on the switch took a lot of effort and didn't even sell a million.  

It sold more than a million on Switch though? We have data from 2020 saying it sold ~700k on Switch. I doubt it sold less than 300k in these 3-4 years.

Call it a million.  It doesn't change the point.  Compare it to mario Kart, ac, Odyssey, etc. 

If there were money to made by switch ports, third party would do it.  There is a reason they don't.  

Witcher 3 launched in 2015 on the ps4 and 2019 on the switch.  In 2019 the ps4 version out sold the switch version 2.5 to 1.....  despite being on the ps4 for years and being new on the switch.  

https://www.gamespot.com/articles/witcher-3s-sales-by-system-how-the-game-has-sold-o/1100-6475832/