I'm entirely unconvinced by your argument regarding backing up a perfect forward secrecy chat. PFS is about preventing any listener of the messages in-transit from ever effectively decrypting those messages - it says nothing about security guarantees once the message is received.

It'd be like arguing that by enabling PFS also creates a social contract that the receiver guarantees that their device isn't compromised. Or that webservers that use PFS in TLS don't log most/all of the details for the requests they receive. I don't believe either of these to be true.

You seem to have extrapolated some social properties from a purely technical property and assumed everyone must think about it in the same way as you. But in doing so, you've made your protocol weaker against realistic attacks.

Oh, and the likelihood of an attacker stealing a device with WhatsApp installed and only being able to extract the identity keys and not the cached messages seems absurdly improbable.

Personally, I wish you'd either tried to work with OWS/WhatsApp/whoever to integrate in what makes keybase.io great (providing identity verification - something that all these could use) and not gone about adding to the already crowded area of chat providers.