Maybe I haven’t thought this through all the way, but how will smart caching relays drive out clients that use the outbox/gossip model? Why can’t they exist side-by-side in the long term?

The scenario you describe is essentially what has happened to email, although the driving force in email was spam. My hope has always been that on Nostr we can solve spam in a more nuanced and decentralized way. If we can’t do that, then centralization feels inevitable. But if we can model trust/reputation well enough to solve spam it feels like those same tools could lighten the burden of running a relay, keeping small relays around.