Is it possible to have truly persistent storage in a PWA and why may you want one?

The answer is No. The user can delete the PWA (if installed on Android) and clear their browser’s cache of everything from your PWA.

The Cache API is temporary by design, and the IndexedDB data can be wiped when the user clears their browser cache.

So to persist app’s data across PWA installations you should use cloud storage via some API.