Webmention depends on the original source to link back to those comments for discovery though, right? I think perhaps a better approach would be a way to comment on any URL and see comments from other users (or communities) that you have subscribed to. That way the author of the content being discussed is not in a position to limit the discussion to what they approve of and you can discuss things that are not even opted into this system.
Ideally this system would also be integrated into browsers you you can see and write comments even when visiting a URL directly.
Thats exactly what my vision is for comments on Haven[1]. Without webmentions, you're just telling your friends/followers what you think which enables private comments/conversations on public content.
Ideally this system would also be integrated into browsers you you can see and write comments even when visiting a URL directly.