I actually think this demonstrates the exact problem. The reason you can't do that at the email level is because Google has taken over the entire email space. Ideally, we don't let threads do that.

Nowadays you have services like Outlook blocking emails Tutanota for "spam protection". I'd really rather threads not get so big that they can start dictating how the Feduverse operates in that fashion.

As I said down thread, the easiest way to get information with the structure of activity pub is to have a bunch of users and that is because the only way information gets transmitted to a server is by caching it based on post interactions.

That is exactly what millions of thread users in the Fedeverse would accomplish.