Unprecedented for a zelda game. It's crazy, I thought that after Skyward Sword the franchise was dead, but here it is competing with Mario for the best selling single player game on the Switch!
That said, I predict Odyssey will beat BoTW when it's all said and done because:
1. Mario games always always still sell well 3,4,5 years after launch. Odyssey I feel will have that.
2. Zelda BOTW 2 will eat away from the original sales. Unless Odyssey gets a sequel, I see BOTW sales trailing off.
STILL, what an achievement! Very well earned for a gorgeous game.