How do you imagine this working:
I bought AppX from the Apple store, I own it. AppX is then removed from the Apple App Store and moves to Steam, how do I get updates?
I imagine I will need to join the Steam store, give Steam all my info including a CC, just to receive updates for an app I already own. It then gets better, 1 year later Epic decides to give that dev a better deal for exclusive distribution rights, now I need to sign up on Epic's store, give them all my info, etc.
My other choice is to abandon the app.
I don't view either of these choices as an improved experience over what I enjoy today.
I have said this multiple times, if as part of the Apple Dev program apps must have a presence in the Apple App store and use Apple for payments as well as be available in other places with other payment processors I no longer have an issue and I think this makes everyone happy. Buy from the Apple store if you want, buy from the devs store if you want but all apps need to be in "both" places.