The chain of messages can be forked in double ratchet. However, any message has a "previous message" like any commit in git has an ancestor. This provides important context, as to understand a message you need to know what previously transpired in a chat. If your attacker doesn't know whether he has all the messages neither does the legitimate recipient and will as such have trouble comprehending the conversation. Your "it's not a bug it's a feature" doesn't work here.