How? If alternative stores are forced on Apple, how can someone have that same locked down experience and still get Fortnite? What about when Adobe decides to make their own store, how do you get Photoshop and keep that locked down experience? What happens when Epic buys exclusivity on new apps (like they do on PC with Borderlands 3) and you absolutely cannot get it if your phone is kept locked down?
This irrationality that another or several other means of downloading iOS apps is going to be some sort of security armageddon amazes me. The Apple store doesn't go away; if you want to only download there, you would be free to do so. Others who fancy it or are not concerned wth the supposed risk would be free to use their choice.
The REAL reason Apple doesn't allow this, is that they know the costs of hosting would be driven down and their cut reduced significantly or they know AAA apps would be elsewhere and wouldn't be available in the Apple App store if there was somewhere else to get them.