APK isn’t a closed source format just because Google operates the main store.
If there was community effort someone could spin up their own snap store, this person did it forum.snapcraft.io/t/…/27109 - problem is, it would serve no benefit because you would have to create your own signing authority and patch snapd to use those assertions instead - and then you are still relying on a central authority to vet and sign releases and frankly I would rather have my software signed by canonical than someone random guy operating their own snap store