The issue with Genshin specifically is that the twin you don't pick as your MC still exists in canon as a character. Whatever twin isn't the Traveler is the Abyss Prince/Princess. IIRC the canon timeline has a male Traveler and a female Abyss Princess.

Compare that to, say, Honkai Star Rail, where whichever Trailblazer you didn't pick just doesn't exist in the game.