I think its hard to set a baseline of Switch 2 sales.
How big is the ''core Nintendo audience''? That is hard to answer, given that we know roughly the baseline for a low performing Nintendo home console with Wii U and Gamecube (10-20 million), but that baseline is harder to predict when it comes to a Nintendo hybrid system. It seems going by history that Nintendo has fewer diehard fans that will never stop buying their systems than Sony has with PS for example. Nintendo thus has to hit certain sweetspots to achieve success, good exclusive games, appealing hardware concept, decent third party support, decent price point in order to have a good console generation when it comes to sales potential.