It would be a lot more difficult than it sounds for Apple to allow 3rd party browsers and allow them to be good. Even on macOS now, Chrome depends on internal OS APIs which prevent it (or Electron apps) from being in the Mac App Store -- and for good reason, those things could break at any time. So in order to allow 3rd party browser engines that can compete with Safari in terms of performance etc they would need to make the APIs that are used to optimize them public.