There would only be a line if the message was a "reply" to an older message. Meaning that user long-pressed that older message, and selected "reply". I'd expect

There would only be a line if the message was a "reply" to an older message. Meaning that user long-pressed that older message, and selected "reply". I'd expect that less than a quarter of messages would be replies, and that typically one would only bother replying to recent messages - just a few back in the chronological list. A message just typed into the bottom field isn't a "reply" in my definition, and wouldn't have a connector line.