All the manufacturer has to do is publish an APK on their website. If all apps did this, Google would have no power. It's very easy for a volunteer to host the APK somewhere in an archiving effort. Much easier that it ever has been on iOS.
Yeah, but if you're a new app, Google doesn't let you have your APK signing keys, so you either have to go through Google to get an APK you can publish (with all the resources), or users can't cross-upgrade because on phone storage is tied to the signing key.
