Apple is only doing this because the EU forced them to, so unless the US forces them to as well, it will never happen. App Store is Apple's golden goose and they would never give even a slice of it up voluntarily
Poor destitute Apple, they have to keep the iPhone a walled garden to feed their families. They don't have anywhere near enough money already. The executives are surviving on cabbage soup. Tim Cook will literally have to eat cardboard for dinner if they don't make another hundred billion dollars next year.