You also assume that using that scheme Nostr will (perhaps naturally) get more decentralized and more people will use different relays, but we have no reason to expect that to happen under those circumstances. The incentives are completely in the other direction:
1. people don't want to run servers (otherwise Mastodon would have been a success)
2. people flock to a few relays that provide this "outbox-on-the-server" service for them
3. doing outbox-on-the-server is actually much harder than on the client, and impossible at scale because of relay filter ratelimits, this leads to these servers actually just giving up the outbox model and syncing with some other selected relays, which creates other types of centralization pressure on the publishing side
4. it also creates another type of centralization pressure, which is the specialization of these "aggregator servers": instead of serving a normal relay interface they are more likely to start serving some pre-processed GraphQL queries or something like that to make clients even easier to write for incompetent LLMs
5. now the barrier to running one of these aggregator is higher, which leads to less entrants
You get the idea. If we get to the end of the story we'll have like 2 or 3 of these big aggregators, with 90% of people in one of them (but even if we had 50 that would still be ridiculously centralized and censorship-resistance becomes a joke).
I don't see why we would even get to end of the story though, because why would such a broken protocol be attractive to anyone? What are its discerning features? Why not use X or Bluesky?