The point is that nostr can encompass any level of openness you care for. it doesn't need to be wide open all the time. From that, centralized social media's or the fediverse's functionality is a subset of what nostr can do - with even simpler implementation.

We already have private and paid relays, but they are just thrown into the soup without any differentiation. 

 The problem is that everything is wide open (for real, on protocol's end) in nostr. You couldn't really adjust that.

Still, Don't fediverse could do the same exact thing? Being half open or similar? Because other users from external server still could give a follow.