That's the entire issue right there. When Apple is the market, then they can make it whatever they want. If Apple has to legitimately compete with other "App Stores", then the market will find a lower price than 30%.
If that is exorbitant, what SHOULD the price be?
There basically is not a competitive market for either app marketplace right now, so an accurate price can't be determined. Allow 3rd party marketplaces to be installed as easily as any other app and I'm certain prices will come down from where they are today.
I'd argue the opposite -- one of Epic's allegations is that Google scuttled a deal Epic had reached with OnePlus to ship Epic's own Android app store.
I expect that number would drop to around 10%, for "premium" app stores, such as if Steam or the Epic app store were allowed on iPhones.
But also, I would expect there to be even lower priced competitors. Payment processors charge 3% of sales, so a competing app store could get pretty close to around 3%, if they wanted.
So the answer is 10% for "premium" app stores, and for the app stores that instead only doing payment processing, the price should be 3%.
Additionally, companies like Stripe have made integrating payments _way_ easier than it was back then. Hosting downloads is not difficult either. As noted in the Stratechery article, developers not only give up 30%, but give up a customer relationship and can't even issue refunds! They can't do upgrades, either. It's not just the 30%, it's the flexibility in payment models as well.
I don't know that prices would drop, but more developers would have a sustainable business.
Payment processors like Stripe aren't new, there was PayPal, Kagi, and others before 2008. Handling payments is a non-trivial task for a small developer, the same goes for handling user accounts/data. They're not insurmountable problems but they're a lot more effort than Apple or Google just sending you money when people buy your app.
Hosting is worse than payments as it costs the developer money and keeps costing money. If your app gets a popularity spike instead of getting a revenue spike you get a huge hosting bill.
I think you've got it entirely backwards, more developers have sustainable businesses because of the App Store than without. Payment processing, hosting, managing user data? That shit is easy for big companies or it's an extant expense so mobile app sales aren't adding real overhead. So for Microsoft, Adobe, and Epic those aren't problems. They could easily sell their stuff without the App Store/Play Store and in fact currently do so. For the many thousands of small developers that aren't Epic payment processing and hosting are big costs as is advertising they need to even be found. Before the App Store mobile app aggregators charged money to get listed and wanted bribes to be featured.
Epic et all might see their overhead drop from Apple's 30% to maybe 10%. They pocket the extra revenue and prices don't change for consumers. For many thousands of developers their overhead goes from Apple's 30% to 50-60% doing it in house. So Epic rakes in even more money and small developers get the shaft.
Pretty small? I think Epic, who makes billions and billions of dollars in overall sales (though revenue is unknown specific to Apple devices, but fair to say at least $100M), would strongly disagree with this point.
There were multiple competing stores and sometimes even the developer's own site. The prices were ridiculous, partly because everyone had to implement their own payment and distribution systems. If the software didn't work or there was a problem it was rare to get a refund. Most of the time your contact info was readily sold to "partners".
Apple's cut of sales for the service they provide is pretty small compared to the costs of doing it yourself. There's many of thousands of developers that are enabled by the fact they don't need to host their apps, handle billing, or handle user accounts and PII.
If you seriously think prices to end users would decrease if Apple took a smaller cut you're deluding yourself. If developers are making sales at current prices there's zero reason for them to drop prices even if their overhead drops. Prices are set by what the market will bear and only have their lower bound set by the developer's overhead.