Honestly, most Fortnite players probably have multiple devices already so it doesn't matter to many. That said, based on how popular Fortnite was at its peak, if 1 in 5 Fortnite players decided to switch platforms over this, Apple would notice it in sales numbers. It will be interesting to see if it puts pressure on them at some point.
Lets not forget not only was Amazon refusing to do Prime Video on iPhone and claiming Apple wasn't doing it for them, they also dropped all Apple products on the amazon website to turn the screw and cause Apple some pain.... yet some around here like to keep acting like Amazons 15% was something Apple gladly did because they "got something in return".... yeah, they got their sales back.
There are enough negative stories coming out daily about the App Store, if Epic is able to alienate a small fraction of a percent of users from buying another iPhone this September, Apple will feel it. If there is one thing Tim Cook was known for, it was being a numbers guy (definitely not a product guy)