All Google would have to do is check what key an app is using on a user's device and they can invalidate it. Someone made a youtube vanced 2 (not revanced)? Google can easily invalidate it for everyone using this process.

Even if I misunderstand how the key works and it's more of a signature, it would mean that if you try to download an older app then it could fail bc the signature expired and isn't maintained anymore. You also run into the same issue with not being renewed if Google chooses.

My point is it could effectively be a side loading killer, maybe not right away but the point of sideloading is the independence which this takes away