I don't get the rhetoric behind the idea that Switch will stop selling in 2020 when PS5 and Xbox 2 come out. Nintendo Switch's library of games barely overlaps with either of those two console bases. The games that have made Switch successful aren't the same games that have made Playstation and Xbox successful. Switch doesn't even have a GTA, Red Dead, Call of Duty, Madden, etc. or the 3rd party games that really push PS and XB hardware...and it's doing just fine.
As long as Nintendo's 1st party keeps pumping out new quality titles, and Japanese developers continue to support it (as it's the best selling gaming device there now, outselling the PS4 which launched 3 years before it) along with the AAA 3rd party western game here or there in addition to all the indie content it'll be fine. Nintendo found an idea that I think can be their line of consoles/hybrid that they can just continue to iterate on and that comes down to how it was executed. If the Switch's execution was terrible (essentially the Wii U) I would be worried but we're nearly 3 years into its life and I see no sign of it slowing down anytime soon.
These are the conversations we had before Switch's launch and now we're even having them after its success. I think Nintendo's got it from here. Sony and Microsoft and even Google have expressed huge interest in game streaming, if Nintendo continues to not create powerful enough hardware for all developers, streaming is an option for them. I know that for me, if Nintendo just continued to make newer and more powerful Switches with quality of life improvements then I'd buy Switch 15 if it came down to it. The benefits of the Switch having a shared library rather than splitting up development studios also can't be understated.
The way I see it is, Nintendo has never sold less than 50M consoles across their handhelds and home consoles of the generation. Even with the poor sales of the Wii U and the casual market moving from DS to Smart Devices, Nintendo still managed to sell around 88M units of hardware and I would think they would be pleased with that level of success. Xbox One was just outsold by Switch, but by no means do I think the Xbox One was a failure not worthy of a successor. It's easy to get wrapped up in this kind of talk, I mean look at me writing a book here. But Nintendo will be fine.