Alright, let's talk about interesting things.
Super Smash Bros Brawl sold 2.7 million units back in March 2008. At that time Wii had an install base of 8.08 million units.
Right now Switch is over 7.11 million units in the US, and iwith November+October, there is no way Switch won't have a bigger install base during Ultimate launch.
To this, add the fact that most Nintendo series had a very big boost in performance, just look at Odyssey and Breath of the Wild, AND, the hype for Ultimate looks by far bigger than any others Smash games at launch.
Oh, and of course, 5 weeks of December VS 5 weeks of March...
It's absolutely insane to expect Ultimate to sell less than the 2.7 million Brawl did at launch.