Why? Like, I'm not saying it's actually the case. I'm only entertaining the possibility. If SwSh would have actually sold 10+ mio more had the game be of better quality, that's something that could be true for one of the most massive franchises in the world. Imagine a world where GTA5 was a significantly worse game than it is. Imagine that in that world it sold 50 mio units. Great, right? But in our real world where GTA5 is the great game it is, it sold vastly above 100 mio units.
Game quality is not always directly tied to a game's success, but it is tied to a game's WoM, and that can significantly change a game's sales potential. SwSh had got massive criticism in the weeks leading up to its launch and still isn't talked about all too fondly amongst enthusiast gamers. It's impossible to say as of now, but what
@Koni said could hold some truth. I'd say let's wait for Arceus' numbers.