The problem with backup & restore in Android was that it didn't work reliably. I know first-hand. If the feature isn't there, people won't rely on it, as they may do when it's there but doesn't work.
The thing with locking issues is that Signal issues get so much noise that the content will drown in it. I dislike not being able to add on-topic comments, too, but I understand how they got where they are.
It was a feature in the app to export an encrypted backup to external storage (encrypted with a passphrase of your choice), independent of OS-provided functions. It basically serialised and unserialised its own database.
The thing with locking issues is that Signal issues get so much noise that the content will drown in it. I dislike not being able to add on-topic comments, too, but I understand how they got where they are.