This assumes that every business model has a 30% margin they can give up to an "agent"; in this scenario, Apple. Many, many businesses operate on less than 30% margins.
Because of this, offering an iOS app becomes a loss leader. No, 100% of your user base will not sign up through iOS, but the 30% commission to Apple will dilute your margins on non-iOS sales. The net effect is that any company who operates a direct sales model will see a dilution of their margins, and must bet on the fact that in-app subscriptions will compensate for this in increased sales. There are many scenarios where this makes absolutely no sense.
Because of this, offering an iOS app becomes a loss leader. No, 100% of your user base will not sign up through iOS, but the 30% commission to Apple will dilute your margins on non-iOS sales. The net effect is that any company who operates a direct sales model will see a dilution of their margins, and must bet on the fact that in-app subscriptions will compensate for this in increased sales. There are many scenarios where this makes absolutely no sense.