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.
If these non-public APIs are so useful it feels to me that Apple is acting in an anti-competitive manner. They're allowed to use them for their own applications but deny third-parties the right to sell in the "Store" if they use the APIs. Sounds a lot like MSFT in the 90's...
