The most important changes Apple needs to make would not cost them a single line of code- they just get rid of their policy to require consistent pricing across all platforms, and allow promotion/linking of external (i.e. not on iOS) payment sources. If Spotify could charge +30% for a subscription purchased through the App store, and include a link to their main sign up page where subscriptions are regular price, eve…
The consistent pricing requirement was removed years ago. This is an article from 2011 about it: https://appleinsider.com/articles/11/06/09/apple_backs_down_...
Though if you're forced to add in-app purchases and cannot advertise the other method it seems somewhat moot.