I might see why they think that way, but I'll have a harder time sympathizing when the parent's use case starts being more prominent: what happens when your app grows in usage, gets out of the "niche curiosity" category for the mass public, and people start wanting to use it for "serious" matters?

Not being able to back some conversations up is not an option. It would be very ironic if the answer to this was "well, then don't use Signal, because we don't care", and people who cared about the WhatsApp stuff ended up being pushed into Telegram (which seems to be the only other popular alternative, by a wide margin).