30m - 32.5m Zelda is going to keep on selling. There is a certain percentage of people who will continue to buy Switch and get Zelda as one of their first 3 games or so. My main fear is that I may be lowballing even with this estimate.
Bonus: I think they will make the sequel try to look like the original, but it will be a game more like what the devs want to make. However it will still be built on the same awesome physics engine as BotW. So the game will probably still be fun, but not quite as good as BotW.