Meh, most phones have full disk encryption. For the average person, encryption at rest in signal doesn't provide very much.

I mentioned some of the pragmatic constraints of fully trusting typical Android / iOS FDE to fully protect the confidentiality of Signal messages in another comment above that I would encourage you to read.

That said, Molly definitely isn't designed for the average person's threat model, that's totally true, but it's also worth noting that just because someone isn't aware of a certain risk in their threat model, that doesn't mean they will never benefit from taking steps to proactively protect themselves from that risk.

IMO, security and privacy are best conceptualized not as binary properties where you either have it or you don't, but rather as journeys, where every step in the right direction is a good one.

I'd always encourage everyone to question their own assumptions about security and never stop learning, it's good for your brain even if you ultimately decide that you don't want to accept the tradeoffs of an approach like the one Molly takes towards at-rest encryption.