How much has Apple made? How much value has Apple added to the devs' work? How much would the App store be worth without those devs?
We can play this game all day if you like.
I can share a personal anecdote of when I first signed up for the fantastical app on my iPhone. I downloaded it from the App Store (authenticated with Face ID), created an account via "Sign in with Apple", and subscribed through iTunes. And in the second year, the dev gets 85% of my subscription money.
I feel the value here for a smaller developer is that the App Store allows them to compete on a more even footing with larger developers. They all share the same visibility. Apple provides the tools to facilitate the transaction between the dev and the customer, and I think that's what the 15/30% cut is meant to represent. Yes, the app is all the dev's sweat and tears, but without Apple streamlining the process from paying an app to downloading it to creating an account, fewer customers would have bothered purchasing it because of all the steps involved.
And I think this is what Apple doesn't quite get enough recognition for. They help grow the overall pie by providing a safe and secure marketplace for people to transact in, and the end result is that more people do end up buying apps compared to if there were no App Store and people were expected to say, download individual apps from standalone websites. It's easy to buy and download apps using biometrics (face / Touch ID), you have APIs like ATT and sign in with Apple providing better privacy, you can view and manage your subscriptions in one place, and the absence of piracy means more people purchase apps the honest way compared to say, sideloading on Android.
Likewise, because Apple has the best users (defined as those with more disposable income and a higher propensity to spend), it's not really an option for app developers to simply pack up and ignore the iOS market altogether. That's why companies like Facebook are still sticking around, and why they are badmouthing Apple every chance they get. They know this is their last chance to reshape the App Store mechanics to their favour.
At this point, I think devs need the App Store more than Apple needs them.