On the other hand, it's really easy to imagine that you'd see something like “$9.99 IAP; $7.99 direct from Epic/Amazon/Google/Netflix/et al.”, especially when it's a company they already deal with and don't have a negative impression of.
The big question I'd have is whether that's possible or the other terms require price parity — and whether they'd be able to do something like offer bonus content, rewards clubs, etc. to nudge people toward their own store. I'd be somewhat surprised if, for example, Epic couldn't entice a fair number of people with some kind of in-game skin or other loot which they could argue has a resale value of $0.