I hear you on this and was thinking about it similarly. I just started diving in to Nostr a bit more. The user has the option to join relays that filter or permit users/content. In this way it is similar to joining a user journey in joining a federated sever for the purpose of finding your community. Except less intuitive to do so. IMO the user experience with the fediverse starts small and gets larger as their network grows, with nostr, its the opposite big then small
Pretty sure a twitter like app can be built with the matrix protocol and a slack clone can be built with Nostr.
Curious how a nostr slack clone would perform next to matrix