Is it technically possible for Signal/Whatsapp to use Keybase keys in lieu of phone numbers? If so, how practical would it be to add this as an option?
I wonder how that would work with key rotation. Those chat services do a lot of key management for you during a conversation, while Keybase is more focused on long-lived keys.
End-to-end encrypt all messages,
but only "exploding messages" (coming soon) will have FS.
Therefore, your history will survive, encrypted, except for the messages you choose.
No. And even if it were possible they wouldn't allow it. Their servers are designed to forbid anything different from what they already do. Forget about that.