1. Let's think about what's in the users best interest wrt app capabilities. Wouldn't it be better for users to not be hit by a 30% surcharge when signing up for Spotify thru the iOS app? Wouldn't it be better for users patronizing small businesses if all of the revenue for events booked through that Facebook app went to the small businesses?
2. Let's figure out if it's possible for Apple to still offer a secure experience for apps installed via sideloading (which is one option; the other option is allowing other payment methods in the App Store for in-app purchases). It should be thanks to their sandbox. They're welcome to tighten the sandbox further too over time, e.g. how they relatively recently restricted background location access, since that was rightly considered more sensitive than foreground access.
Apple can easily offer more user benefits without fundamentally making their business model unsustainable. Note that the situation here is fundamentally different from brick-and-mortar stores, which have very limited shelf space; not to mention no single brick-and-mortar store controls so much of someone's life (we're very reliant on our smartphones).