I remember the days when you could run your own SMTP server.

"nostr will end up like email,,," - are you giving up already? What tools are available to make that less likely? Paid relays are should help, but that still means it is likely to centralize over time.

Maybe relays become more focused on specific topics, and you choose the specific ones you want to pay for like a la carte cable.

Then how do you get serendipitous exposure to new content and communities?

Run a relay through a Tor gateway or VPN. Keep Freedom Free!