> Valve has competitors in the PC games storefront market. On iOS, there is (for all practical reasons) no other storefront.
Predictably, this is where the goalposts have moved[1] but it's really orthogonal to whether there's value or not. Also as pointed out elsewhere, Steam is in a competitive market and charges the same commission. Also in Apple's favor is the well attested to value of the App Store vs Google Play and others in terms of raw revenue.
[1] 'Predictably' as in "Being locked down or not is a red herring to the value argument" in grandparent