If something could be built as a native app without depending on a central server, it could also be built as a PWA without a central server. You don't need to store user data centrally at all, just because it's a webapp. You can just have the clients use localStorage or IndexedDB or whatever.

You still have to host the static files for the webapp itself, but that can be made very cheap.

Of course, API feature parity between native and web apps is a separate issue. But the argument about server costs doesn't seem like a good one.