Ah gotcha, yes, indirect replies would need to be forwarded or something.
(To be honest, I'm already surprised that Mastodon scaled as far as it did. I will say, if I had seen the state of the web's architecture 20 years ago today, I probably also would have claimed that it was inherently insecure and that there was no way to get it to be secure enough to scale to billions of users, so... I don't know, maybe people will keep finding duct tape solutions to make it work, worse-is-better-style.)
Imagine if, when you refreshed this HN page, only comment chains you’re already in would refresh timely. Yes, this would “work” to some extent, but it would clearly be a regression.
Additionally, going viral can overload your server due to this architecture. In ATProto this never happpens for self-fosters (of PDS) because the cost is amortized by AppView. (Same as in centralized products where the cost is on the backend.)