At this point the only thing that can stop the Switch outselling the Wii is Nintendo.
If they keeping going the way they are and support the system properly, it will pass 100 million.
They only way it won't is if they go full moron and cut it short prematurely.
People talk about Nintendo not having to support 2 platforms anymore but I think alot of them dont realize that in ~2010 Nintendo was developing/publishing titles for 4 seperate platforms at the same time.
They were trying to provide DS & Wii with late life support while also trying to get first year software ready for 3DS & Wii U. This caused them to be spread too thin and the transition period saw DS/Wii sales drop hard while 3DS/Wii U both got off to slow starts.
The transition to Switch went much smoother as they killed off Wii U early and were able soften droughts by releasing Wii U ports while they kept supporting 3DS as a back up plan if Switch didnt take off.
In theory the transition to Switch 2 should go even smoother since it should continue to use the Tegra line of chips making it easy for developers to transition and cross-gen titles should be easy to do. On top of that they wont need to give unnecessary support to a seperate back up plan since the Switch concept will no longer be a high risk proposition.
So yeah, Nintendo has to do something foolish to make Switch suffer an early decline or Switch 2 to stall out of the gates. Something along the lines of returning to two or more seperate ecosystems or going with a completely different high risk concept for the next hardware or going with a different chipset making it hard for developers to transition.
If they can continue giving strong support to Switch until 2023/2024 then release a Switch 2 in the XBO/PS4 ballpark power range than all should go very smoothly.
When the herd loses its way, the shepard must kill the bull that leads them astray.