A huge launch was always to be expected, because Breath of the Wild's sales were massive. Going forward, I don't see much of a reason why Tears of the Kingdom's legs should be in danger. The reused overworld is offset by enough new mechanics, enemies and locations as well as completely new shrines, so it's feasible that the 20m mark (global sales) can be crossed by the end of 2023.
Legend11 correctly predicted that GTA IV will outsell Super Smash Bros. Brawl. I was wrong.