The signing key is what allows for upgrades that still gives you access to all of the data from the app; if you have an app with a pin code that is protecting a signing key or a password database, this means Google can now help people upgrade the app without it being an official upgrade from the source you chose. You might try to argue that Google can already just change the operating system out from under you in similar situations, but that forgets that Google doesn't actually control Android: the manufacturer of your phone is the only person who can ship updates for your phone (not Google). Even mitigations focused on this behavior for just Google's store are disappointing, as the current signing key approach is designed to better support multiple competing app stores and platforms: your app, as distributed for every device by ever vendor through any store is signed by the same key... that's what makes it "your app". It should be the case that the app is end-to-end encrypted through any store and that any store is then able to securely upgrade the app you for from any other store... this all is eroding that as it is giving Google too much power over authenticity.