Everyone doesn't need to run a relay, but everyone could. Running a relay is already incredibly easy to do, and costs nothing, and that'll just get better. It's not decentralized, if you need to traverse "approved" hubs to get from point A to point B. That's not only a protocol question, that's an implementation question. We are emphasizing a complex, sprawling network of privately-run nodes, preferably communicating offline or over Tor, to avoid the centralized fate of email. Every relay operator is free to let me use his relay or not. This is a nonviolent protocol. We do not need consensus in event content or originators. We just need lots and lots of different paths, and to ensure that anyone can build their own path.

I also think you are underestimating the importance of the timing of introduction. When e-mail began to be popularized, in the mid-1980s, hardly anyone had a computer. So the few of us using it had it to ourselves and it was common to host an e-mail server because only computer geeks were on it (my dad is a network engineer). Then it got really busy in the 2000s, but running your own server was a big technical hurdle, to most people, so they were happy to outsource the work to Google and etc. and that allowed for network consolidation, by creating email bottlenecks, where they would filter spam and etc. Now, anyone can create their own, personal intelligent spam filter and run a relay on their smartphone or laptop, just by downloading an app. That's a game-changer.