It’s easy to imagine new services being developed to sit between relays to deduplicate content or otherwise optimize, or (even more likely) relays that are also clients of other relays for the same purpose. Either way I think it’s fair to say that a partial mesh or inter relay routing is necessary for nostr to scale to 1B, but maybe 1B is not necessary. Could be that smaller communities like an enterprise whose relay is behind a VPN, or topics/community-based relays will be the final form