@ricmac @Gargron so, I am not sure I agree. I think there are plenty of Fediverse actors that won't follow others -- especially automated tools. So, Fediverse-enabled data sources or IOT devices.
But I think we will also have a lot of publish-only applications, which accept replies and likes, and allow replies to replies, but don't have an inbox or subscriptions for authors. CMSes, blog and newsletter platforms, etc. There are a few that work this way already.