A fee to be on the App Store and have apple host your application on there, with all it entails, is perfectly reasonable. But this is about leaving the App Store. For your analogy to make sense, the iPhone as a platform would have to be Costco, and last I checked I already bought my iPhone. I own it and should be able to put any software on it I want. I am not renting an app platform. Apple has even themselves said that they don't want to be a business where you are the product. They want to sell products. Well, in the same vein, once you pay the product you should have control over it. I'd be happy for the default setting to be App Store Only. I'd stick to that myself. But on principle I feel like direct software distribution ought to exist. And that doing so should be able to circumvent Apple entirely. A Gatekeeper-like system would be fine, including the Apple Developer program fee, but even that should have a "Forced" 'launch anyway' option as it does on macOS.
I've always thought the 30% fee on the App Store is reasonable. I'm happy staying 100% within the App Store. But I also fundamentally believe you should have the right to run anything that can be run on your device.