IMO what cut Apple deserves is the wrong question. A democratic society can in principle decide that the public would be better served if Apple had to allow alternative app stores, and enact legislation accordingly. Whether Apple deserves to establish itself as the sole gatekeeper and take a cut whenever they can manage isn't really a concern. I think using anthropomorphic language like "deserves" for a company isn't even conducive to thinking about this.
I'd ask this instead. Would it be beneficial for society as a whole if iOS had to allow alternative app stores?
Why should Apple be forced to include an app store on THEIR operating system? If they want to be the gatekeeper of all their software, it is on them as a private company to do so.
It is likely beneficial for society as a whole for iOS to allow alternative app stores, but that doesn't make it right for the government to force them to do so.
If the citizens of a particular country want to force Apple to allow alternative app stores, they can. It's their country. Apple can ignore their market if they don't like the rules.
I'd ask this instead. Would it be beneficial for society as a whole if iOS had to allow alternative app stores?