I can't say I see many people on here predicting total sales to be less than 100m... I think most know that it's a given at this point (barring asteroid strikes or deadly covid-21)
Those that do perhaps just don't understand sales momentum, the Switch is technically behind the Wii in launch aligned sales after all and the Wii only just topped 100m. Maybe that makes things look bad for Switch, but that's just a snapshot in time not looking at the bigger picture, Wii may be currently ahead launch aligned but it's average non-holiday sales months were down to less that half of it's peak year.
Momentum is important, the Wii sales exploded from launch but peaked in it's 2nd year and was already on the decline in early 2009. Switch is still accelerating coming up to the end of 4th year.
It's fairly easy to predict a few months in advance, +75 million is the minimum it will be at by the end of December (78m more likely). If from January onwards it has the rapidest sales decline of any console/handheld in recent history it would still break 110 million easily, and a more modest Wii like decline (which was still very fast) would see it topping 120 million.