"A native app would do just fine", why would we want to build a native app using proprietary tech, be forced to go through an app store, risk to be turn down by Apple and be taxed 30% of our revenue otherwise? All this when we can build web apps using open standards, working on all platforms and without being extorted by a giant abusive conglomerate?
You would want to because you want access to Apple's customers. Also, your example doesn't sound like it'd hold up to real world practice. If you're selling someone proprietary tech then your revenue isn't subject to 30% of Apple's fee. Only the software portion done through the app is. You obviously have a motivated buyer at this point if they bought a physical device. You could route them to a website to create an account, sign up for any subscriptions and launch the startup process that eventually has them install your native app. You want what you want, but you describe hyperbole. Plenty of apps have users manage subscriptions on a website, not on native apps.