We estimate that by 2025, Signal will require approximately $50 million dollars a year to operate—and this is very lean compared to other popular messaging apps that don’t respect your privacy.
Signal provides a backup option. The auto backup for SMS on android is provided by google and likely uses google drive. I don’t know for certain but I would guess the encryption options and security of that route would be impossible to guarantee and the public backlash of signal users knowing their data was being sent to Google’s servers would be massive.
I’ve setup my signal backups to a local folder on my phone. I then have SyncThing running on my phone and home computer so it automatically gets sent once it’s created.