I guess we'll just have to wait and see. Wait for about a decade until after the main Zelda after the BotW sequel has released and settled but whatever.
And you shouldn't, because Nintendo has shown enough times that they don't make games based on rational business sense.
Not even Breath of the Wild 2 is a safe bet for 15m+ yet. We still have to see its overworld, because up till now the trailers have focused on floating islands in the sky. There shouldn't be any doubt that the overworld and the ways it can be interacted with is the key to Breath of the Wild's huge success, so if for whatever reason Nintendo decides to limit that aspect, BotW2's sales have a good chance to be disappointing, to the point that the original game will keep selling at a higher rate than the new game despite being six years older.