The thing you're ignoring is developer reputation. Google (or any app-store runner) doesn't want to allow you to use parts of their service—for which they could be held liable if those services take your money and give you nothing in return, or give you a virus—unless you're also using their identity system and thus their developer-reputation system.
And if you are using Google's identity system—in what way are you not just back to publishing on the Play Store? Through the identity system (and associated developer-reputation system) they can control who can or cannot publish on your app store. So it's really just their app store. It's your UX, but it's their guiding hand, just as if Google were on your board of directors.
Really, there's no point in them letting you have your own app store which relies on parts of their infrastructure, since they can tell that any implementation their legal dept would okay is a lose-lose that no dev would ever be interested in taking part in. To detach any liability for the results from them, you have to do it all the app-store stuff yourself. Which is exactly what mobile OEMs do.