> Because it seems to me that, for much of Signal's (often paranoid) audience, they'd much rather use one of the backup/sync providers they've already verified trust of, than have to additionally trust some new backup service provider.

I wonder if those folks are the ones actually asking for backups. Personally I couldn’t give a shit about my messages from more than a week ago and part of that has to do with privacy. People concerned with privacy might also be more likely to have disappearing messages on which is the exact opposite of a backup. I honestly wonder who this is for exactly. From reading other threads, are people using this as their primary media backup basically?

This. The former core user base would prefer the definitive history destruction to a reliable backup. But this audience doesn't grow this fast anymore.