Legacy apps where the devs have the keys certainly have an easier time migrating off Play Store. They "only" have to convince their clients to enable "untrusted" sources and download updates from their websites. Users can then update, knowing the binary was signed by the same devs as before.

New apps essentially have to start from zero and tell their users to install that other app if they loose their Google Developer account. Clients cannot upgrade from the Play Store version to the free version. They have to either delete the Play Store version before installing the free version or the provider has to provide the free version under a completely different app ID.