To be honest I find it extremely hard to believe the Switch will fall short of 100 Million, here are several reasons why:
-The Switch is over 3 years old now and Sales Year over Year are STILL growing, as Switch sales are up an insane 43% in 2020 compared to the same time period in 2019, compare that to the Wii where the Wii within 2.5 Years into it's life-cycle started seeing declining Year-Over-Year sales which lead to the Wii having a much shorter lifespan in sales. The Switch still growing in sales over 3 years in is showing how the Switch will have a much longer lifespan in sales over the Wii, and the Switch is doing it with literally ZERO price cuts thus far, while the Switch Lite is cheaper most people still prefer the Regular Switch over the Lite. Once the OG Switch gets a price cut it'll increase the Switch's lifespan even more.
-Plus the Switch is a a rate of selling nearly 28Mil in 2020 alone, while the 43% growth in 2020 is likely inflated by the release of Animal Crossing, even a more likely 25% growth would result in the Switch selling 24 Million for the year. Making the Switch sit around 75 Million by the end of 2020, pretty much borderline outselling the 3DS's lifetime sales. The Switch sales need to die really quickly after 2020 for the Switch to somehow fall short of 100 MIllion, die to a sales amount that's unprecedented which is very unlikely.
-The Switch will also get Nintendo's FULL software support for the first time in Nintendo's history, meaning the Switch will be full of releases with little droughts for the next several years of the Switch's lifespan. And games are what sell people to game consoles the most, and the Switch will have a ton of them for the next several years making the Switch very appealing for all gamers to purchase.
-Also since the Switch is essentially a handheld, I wouldn't be surprised to see Nintendo try to milk the Switch as much as possible by releasing a multitude of Switch models to keep interest and sales momentum up. They did this with the Gameboy, DS, and 3DS and all were benefitial in maintaining the hype and buzz around those systems and a good amount of people purchases two systems in the same family, increasing overall sales. I'm sure a Switch Pro, maybe a console only Switch would help the Switch keep those sales up for a very long time.
-Also since it's a handheld, the Switch will age like fine wine no matter how anemic the hardware gets, usually consumers don't care much about graphics when playing portably. The 3DS, Gameboy, and DS were all very technologically inferior compared to their competitors even when they first released but they all killed the competition. However, the Switch I believe will age far better than those systems since the Switch when it first released had graphics comparable to Current Gen dedicated Home consoles like the PS4 and Xbox One, the Gameboy, 3DS, and DS often times had graphics comparable to last to 2 generations back. The Switch in 2030 might still be a competent experience.
-Also, Nintendo essentially has a monopoly on the dedicated portable console division now after killing off the PS Vita, if you want to play console like games on the go, Switch is pretty much the only option to go to.