They arent taking that away. No one is going to force you to sideload because most people won't sideload. Only a few Android apps aren't on the play store even though sideloading is very easy.
and apps that have their own Stripe account and/or want to tell customers that they can save 30% by paying through the website, or pay 30% more to use the Apple subscription/payments system. But user education and market competition would mean Apple's 30% is way, way too much, so hardly anybody would actually do that if they had the choice...