If they allow PWAs to get around the 30% IAP feature, then in 5 years there will be no more native apps, only PWAs. I doubt they will allow any non-google play in app purchases to take place.
I think the essence of a web app is that it works on any platform. It shouldn't know anything about Google Play Store or Apple App Store. Otherwise, it's just a native app with a webview.
They are called PWA, not WA, with P meaning Progressive.
The idea is that the app should check for available browser/OS APIs that progressively enhance user's experience.
And no they aren't necessarily just a native app with a webview, on UWP a store delived PWA has full access to the UWP APIs, just like .NET and C++, no need to write extra FFI manually.