If you signed your message with "This is a reply to msg 33398198 by Timja. Signed: twblalock", then nobody could take this converstion away from us again. I could copy it and put it on any server. Owned by myself or run by some service provider. And it would always be clear to everybody that this conversation really took place between the two of us.
In fact, users could use any service they like.
It would not matter what client you use to reply to me. Your reply would appear on Hacker News, because HN would follow the protocol and accept properly signed comments relayed to it.
In your example HN would still need to retrieve and store billions of messages, handle user authentication, discovery, aggregation, additional data handling (e.g. you want to attach a video, or go live). They will need to monetize _something_. So we are back to square one, just with a much more inconvenient client setup process (like we do right now setting up pop3/smtp/imap).