@danlyke @davew OStatus = ActivityStreams + Atom + PuSH/WebSub, it's what Fedi ran on before 2019.
Mastodon switched to ActivityPub mainly because of a simpler protocol stack and explicit support for scoped addressing, which enables DMs. This caused other Fedi applications to move as well, as well as leading to a cambrian explosion of entirely new applications.
So while using the equivalent of RSS for all this is clearly possible, it was leaving that behind that created the network we see today.
Still, Mastodon does offer RSS feeds for all users: mastodon.social/@clacke.rss