I agree that without decentralization, nostr becomes uninteresting. So centralizing on the same big relays is not an option.

The only alternative to outbox/inbox that I know of is blasting events everywhere, and that has the worse downside of massive resource usage and its scalability becomes more and more difficult the larger the network grows.

I think the downsides of the outbox/inbox model aren't a big deal.  They only arise on mobile if you have low battery and are trying to follow a god-awful number of people, or if you want privacy but for some inexplicable reason aren't operating over tor and so you are pissed that your IP address leaked to a server you didn't pre-approve of (even that can be solved with whitelisting).