That would certainly be an interesting turn of events if major apps like Facebook or Whatsapp decided to d-list from the app store and require their own store. Given Apple's current heavy fees and disincentives to do that, I'm guessing epic is probably the only company that will. I think a lot of people would refuse to install an alternative app store to get most apps.

But most of all, I look at Android where alternative app stores are possible with essentially no cost, and even with Amazon's enormous resources behind it, they could not get an alternative app store to catch on. I can't think of any reason why it would be different on iOS, so ultimately I suppose I expect this to go nowhere and a few years from now there will be the epic game store and the main app store and everything for nearly everybody will just be on the app store like it is now. The alternative stores will just have the apps that Apple wouldn't approve, like porn apps or privacy invasive, etc