First, federation is complicated. Go look at the size of the ActivityPub spec to get an idea of the complication that federation requires. Sharing messages between relays is not as simple as just sending the messages to all other known relays. At minimum, every relay would have to keep track of what messages it has sent to which relays, in order to avoid relays spamming each other with 100k copies of the same message.

Second, clients can easily connect to dozens of relays. That's one reason why WebSockets was chosen.

A relay directory that knows all public relays is unnecessary, because users can trivially tell each other, "I am connected to these relays." Moreover, it is not desireable for every client to connect to all public relays. Because in 10 years there could be 1 billion Nostr users, and no relay will ever be able to handle 1 billion websocket connections.

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But Lopp concerned about something that is going the way of fax and chatrooms. Lopp is a social media influencer, which is source of income for him. And to do that job, he needs to know how many people are watching him. But Nostr is not a social media protocol, so social media influencers will not thrive via Nostr. A change of perspective is needed for people to properly take advantage of what Nostr provides (which is so much more/better than social media). Social media is a technological dead-end, like chatrooms, forums, email, and fax.