I think this model looks at apps as widgets too much. People don't price compare IAPs. Nobody's like "well fortnite charges X for Y, but Minecraft charges A for B." App store revenues are probably largely games or similar things that don't really have a competitor you can easily price compare against.

App developers set their prices at whatever maximizes revenue (which is often determined by testing) and the payment processing fee has no effect. It's more like Coke pricing at an airport than Coke pricing at a gas station with another gas station next door.

I expect they'd offer whatever discount maximized revenue, so the real question is how much of a discount do they need to offer to get you to switch from app store payments to some other method? Other methods are largely pretty seamless these days too, so the answer is probably not much. I expect consumers would barely benefit.

(Source: been doing games with IAPs for 13 years).