Their mobile effort will see about as much success as any alternate store has in the Android land. Despite being huge Samsung has managed to do jackshit with their store. Of course Google did everything they could, legal or otherwise, in order to keep everyone on the Google play store, but at this point only the end result matters.
Despite all the recent EU rulings and whatever else, it would be naive to think that Apple will not do everything they can to hamper all the alternative store stuff too, again legally or otherwise.
Plus, globally speaking Apple mobile devices are something like 25-30% of the entire mobile market, even if Epic somehow manages to make the store a hit, it's hard to see it being important in the grand scheme of things.
And of course, Fortnite was never that big on iOS in the first place, that's not going to change.