Hypothetical progression:
Epic sets up alternative App Store which becomes the only source of Fortnite 2.0, which is popular enough to drive installs of new alt store. Some other newly-popular viral app comes along (probably a social app that causes its users to exert pressure on their social group to join) and gambles on going with the Epic store only, piggybacking on Fortnite’s beachhead. Repeat. This process normalizes an alt store, forcing iOS customers to have to choose between trusting a new source vs missing out on apps. They can currently miss out on Android-only apps, but at least currently 99.9% of apps that run on iOS come from the single “trusted” store.
Another maybe-more-realistic version of this would be if some business software used their power over line workers to install things (since end-users are not really the customers in corporate software sales). Microsoft sets up their own store as the only source of Office Redux, and millions of iOS users are forced into this store via their employers. Same process as above: beachhead, gamble, dilution of Apple’s store.
It’s not hard to see how the process could snowball, though it’s certainly not guaranteed as there must be the right combination of events.