I think we should also keep in mind that there‘s a good chance that TotK will sell less than BotW, perhaps significantly so. That‘s a pretty common phenomenon with big Nintendo IPs, and is probably one of the reasons why many of them only get one game per generation. Skyward Sword sold significantly less than Twilight Princess, just like Mario Galaxy 2 sold significantly less than Galaxy 1, or like Majora‘s Mask sold significantly less than OoT. And I have no doubt that, if IPs like Mario Kart or Smash Bros had more than one game per gen, that the subsequent entries would sell less than the first ones. This isn’t the case for all Nintendo IPs, but certainly for the big ones.
Furthermore, BotW was a very unique game that launched alongside a new system, and was also the very first open world (3d) Zelda game. TotK on the other hand will be a direct sequel, and the novelty of BotW‘s unique gameplay systems has worn off by now. If TotK ‚only‘ manages to sell 20 million copies for example, as opposed to BotW‘s almost 30 million, then it wouldn’t be that hard for Hogwarts Legacy to overtake it imo.