What happens when other companies see dollar signs and do the same thing? Nintendo is really good at testing the waters. When something works, other companies follow suit. When it doesn't, they fail miserably. In any case, nothing is stopping other companies from taking any idea, tossing more money into it, tweaking it, and doing it better than Nintendo.

We all know Nintendo. They always want to try something new. I wouldn't be surprised if the Switch gets replaced by a completely different idea. So there's nothing like a guarantee.

Nothing is guaranteed in this industry, that goes for all brands. They've all had blunders after major successes

Assuming the next system is Switch 2, similar system but vastly upgraded, I think they'll do 100million again. But beyond that its impossible to say will technology will lead. By 2030 they could be trying out some new technology and interest in it fails so they have another poor selling system. Who knows.

As long as Nintendo stays with hybrid consoles, they are guaranteed success.  Of course, I don't put it past Nintendo to stop doing hybrid consoles.  That is the one way they could screw things up for themselvs, and I do think they are likely to keep making hybrid systems with each generation.  But I've also seen Nintendo turn away from the guaranteed successful route enough times to know that they might do it again.

So, assuming they stick with hybrid consoles, then I don't think they will ever sell less than 85m no matter how they screw up anything else.  Nintendo has never been defeated in the handheld market.  Sony gave a real good try at it and so did Sega and a lot of other companies.  Nintendo just can't be beated in the handheld market.  The worst screwup they had woud be Generation 8, and 3DS + Wii U was still around 88m.  That is why I say 85m is the absolute floor on a hybrid.  Even if they do everything wrong, like they did for Generation 8, they are still going to sell at least 85m.

Not guaranteed, but relatively close to a guarantee.
Every Nintendo handheld (minus the bizarro tabletop Virtual Boy) has been a good to earth-shattering success. Nintendo's home console lineup is much more inconsistent. The Nintendo 64 and GameCube sold well below expectations and the Wii U was an outright failure.
A lack of first-party games people are interested in or a high price could keep consumers away from a Switch 2 or 3. Then they are some other factors like marketing/brand image that could tank a platform.
I think Switch 2 sells 60 million in a worst-case scenario. Switch 3 could be an outright failure.

If their next system is just a straight Switch 2 I think it is extremely unlikely it will fail, but as others have pointed out, Nintendo do have a habit of change for the sake of change which can sometimes be disastrous. We'll see. At least for the moment they are thriving and that should hopefully hammer home to them that they're onto a winning formula and should stick to it.

the amazing thing about the switch is, that they can innovate on the control scemes without losing the base product

Besides simple improvements like better build quality, I think they should just do a regular iterative successor to Switch; same hybrid design, just a bring it up to 2022-2023 mobile tech. No lame gimmicks like the 3DS or Wii U.

Nothing is guaranteed. Just look how the sales went from Gamecube to Wii to Wii U to Switch.