I think both games are pretty amazing in sales, I did not expect Odyssey to surpass 15 million based on past 3D Mario sales. I am not as surprised to see BOtW surpass Odyssey, Nintendo got it really right with that game.
I believe those expecting BOTW 2 to cut the first game’s sales are right, but let’s not forget that it was a one minute teaser. I don’t expect it to release before Christmas 2020, and it could go even later.
edit: surprised we are past 15 million for Odyssey, let alone 16.