I kind could see it happening for real, but once BOTW 2 came out BOTW will slow down. Not at first though. At first and like 2 quarters I guess the BOTW will actually have a boost in sales thanks to the hype and buzz of the new release, and specially if there is a bundled version of both games with a healthy discount like 100 dollars for both
However once that hype vanish, unless the sequel end being very underwhelming I don't see newer Switch owners buying BOTW instead of BOTW 2, so yeah, even selling something like 7 million in 2020 and 2021 combined (26 million at that point), I still don't see it selling another 9 million in the last Switch days (2022,2023,2024) thanks to cannibalization against BOTW 2
I agree with your 30 million prediction though
One thing to keep in mind however, is that people will not buy BotW2 in place of the first one. BotW2 could actually drive sales of the first game for those who did not play it. Maybe not right at launch, but as they finish BotW2 many who started with it will go back and play the first game as a prequel in their own minds. Especially being a direct sequel, most people would want the full experience (both games).