I can imagine app-developers that do no want to pay 30% of their earnings to Apple, opensource apps, alternative browsers like Brave, Firefox, Chrome/Chromium, game developers like Epic, and others that cannot or don't want to provide their apps on the standard app store, will be exclusive to third-party stores. If some of those happen to be killer apps or start trending, then maybe it's possible Apple users will take a step outside their walled garden.

It's entirely possible Apple will try to make the third party app store a nigh intolerable experience or make advertisements and officials claims that it's safer without third-party stores or whatever. Standard corporate bullshit.