Let’s start the list of UX that could be built if apple followed these rules starting tomorrow. What will be available to me that I don’t have now, and how will it benefit me, Joe P. Consumer? Keep in mind, I have no idea what a software developer does all day — I just want my email to work and these internet pop ups to be easier to close. What am I missing out on?
It may allow for some healthy competition and ultimately better UX on that front in the long term, but in the short term it could also take away dev time for Apple to improve iOS in other ways.
The mobile phone platform has been "complete" for years now. There are little niggles to work out here and there, but the single most meaningful change that could come is the switch to an open platform. Consider the abundance of tools on desktop platforms like macOS that hook into the system to augment the UX for the better. Imagine having to use macOS without some window management tool like Rectangle, or without a key rebinder like Carabiner. Personally I would find my user experience greatly diminished. But that is the status quo on mobile. Opening iOS up to extensions and third-party integrations would allow for much faster development of the platform's UX, assuming that Apple Sherlocks whatever extensions end up popular. I have no confidence that Apple will be adding useful interaction paradigms on their own (see Stage Manager) so this seems like the inly path forward.