Firstly, you're assuming that app store sales number would have been equally successful if iOS was a fully open platform. Apple did a damn good job of converting iPhone customers into iOS software customers when compared to Android. Without the App Store, rates of conversion wouldn't be as high.
Secondly, you're assuming that Apple would have put nearly as much effort into the iOS platform, developer tools, software support and encouraging its customers to buy apps, if Apple wasn't seeing a revenue stream from it. No revenue stream, no incentive to build that marketplace, fewer sales.
Third, you cannot make assumptions about how prevalent piracy would be on iOS if the platform was open like macOS, and how many sales would have been lost as a result of piracy being widespread. Historically, levels of piracy in PC games have been in the order of 85 and 95 percent.[1] On Android, piracy has been estimated anywhere between 50 and 90 percent.[2] Of course we know that the vast majority of pirates wouldn't have paid if that was the only option. But assuming a piracy rate of 85%, it doesn't take many pirates being turned into paying customers to net more revenue than Apple takes in App Store fees.
Scenario one, "fully open iOS":
85% Pirated
15% Purchased
80% Didn't purchase
20% Purchased
[1] https://www.youtube.com/watch?v=2MIEVvv1QP0
[2] https://www.makeuseof.com/tag/piracy-android-how-bad-is-it-r...
Second, we have Java based, side-loadable Android universe as a reference point to determine rates of piracy. We also have desktop.
While piracy is always some kind of problem, it's generally not a problem that has wiped out companies. Adobe does just fine, in fact, the piracy threshold might be a reasonable 'valve' for people who don't actually have the ability to pay, to use the app. Adobe benefits from this. So long as people are truly 'not going to be customers' then people using the software is mostly good.
30% is nowhere near approaching a reasonable fee to 'defend from piracy'. It's not really even part of the equation frankly.
As for justifications, you've completely misunderstood my point. I'm not claiming that Apple should justify its 30% on piracy. Let me put it another way. I reject the assertion that an alternative universe where the iOS marketplace wasn't a walled garden would have reliably resulted in higher overall revenues for developers.
Obviously, if you wanted to propose a situation where everything was exactly the same but Apple took a smaller share of revenue, developers would make more revenue. But for most developers this hypothetical is now real, since all but the largest developers can now receive 85% of gross revenues.
There is one area where I disagree with Apple's policies, which is how they only allow in-app purchasing for multiplatform "reader" content. Companies like Netflix and Spotify should be allowed to subscribe customers using their own merchant system within their app, so long as the subscription is entirely multiplatform "reader" content.
The reason why I disagree with Apple on the above is because it's simply too dangerous to stand this ground. If Apple wants to be in the business of selling music and television, they need to suck it up and compete on a level field. Their current stance looks too much like penalties for Spotify and Netflix as protectionism for Apple Music and Apple TV+.
Google and Apple now offer a 15% rate to the overwhelming majority of developers. Whether you call it a response to the market or a cynical ploy to stave off Government intervention, it means small and medium developers can no longer be seen as a profit centre for these mega corporations.
Apple actually has an additional compelling argument, which is the near-total elimination of software piracy on the iOS platform. I don't know what the real-world numbers are, but I honestly wouldn't be surprised if iOS developers ended up earning substantially more from the absence of piracy than they lost paying a 30% commission to Apple.