Yeah that is fair. That assumes that the developer passes the cost on to the user and the alternative app stores take 0-10%.

Here is my fear. I like most, not all, of the restrictions the App Store puts on apps. If there were alternative app stores developers might only provide their apps on those stores that don't provide the same user protections I value. This is why I would prefer Apple to resolve this without allowing other app stores. I think this could be done by adjusting their rules and reducing the amount they take.