You seem to be missing the point here: this isn't even about storing your data on someone else's computer with some kind of key escrow, this is about local backups not even working. Apple only recently implemented iMessage "sync", but before that (and still now), iMessage data was backed up to your Mac and accessible in your backup, without any concern about it being on some server or key escrow issues. Signal is simply missing the ability to get your own data out of the app on iOS. (And like, to really underscore how this is not a fundamental issue with Signal, their Android app does have a data export feature. They just don't think this is important enough to prioritize for some crazy reason.)