The best case outcome is to get publicity leading to US and EU antitrust regulators to file a lawsuit against Apple, both of which Apple loses. The conclusion of this lawsuit is that not only must Apple allow access to iMessage, they also must allow changing the default for every component of iOS - messaging app, browser, app store, let you replace Siri with other voice assistants - and to lower the 30% app store fee to 10%. Same rules apply to Android.
Okay, that might not be likely, but you did ask about the best case outcome.
Okay, that might not be likely, but you did ask about the best case outcome.