The 3DS Brain Training flopped hard. People have moved on. There are a million of these types of things available on phones for $0.
I figured you would say that but as I've been telling you since these discussions began, the Switch cannot become wildly successful without the mass market customers on board. For the holiday season we have Pokémon, Ring Fit Adventure, Brain Training and the Switch Lite and the first big game next year is Animal Crossing. Nintendo is targeting a broader audience with the Switch now and this is a good thing: Switch sales have been very decent but nowhere near the baseline Nintendo wants them to be... because it targeted too narrow an audience before. This is the way to go forward and this time Nintendo will hopefully avoid the mistake of releasing half-baked games for the mass market, thus avoiding low sales.
Will Brain Training be as successful as the DS version? Certainly not. Could it sell millions of copies? Sure, if Nintendo plays their cards right. And right now there's a very good balance between hardcore, classic and casual games on the Switch. It's a win-win for everyone.