Even if you disregard the eBay policies, even if you assume that some people just have to play on iPhone for some reason, and even if you disregard the AppleID issues...
I still just don’t see how this could possibly be a long-term situation. Epic and Apple will go pound their chests in court, and either someone will win or they will settle. Fortnite will probably find its way back to the App Store, and even if it doesn’t, players will just move on to some other game, diminishing the appeal of Fortnite. And as others have mentioned, it is unclear how long the existing installations will remain functional anyway.
One way or another, I would say that there is a 90% chance that having Fortnite preinstalled will become irrelevant within a year.