Switch has enough life left in it to make this a 40m+ seller by the end of its run.
People have to remember that Super Mario Odyssey crossed the 25m mark already and SMB Wonder is simply a bigger deal. Not to mention that NSMBU Deluxe sold more than 15m copies already and that's just a port of the final game of the NSMB series that had become stale by that point.
That's why it's weird to see a bunch of people say 20m+ in this thread, unless they all mean 20m+ by the end of 2023.
Legend11 correctly predicted that GTA IV will outsell Super Smash Bros. Brawl. I was wrong.