... the truth is that sender reputation is by far the biggest indicator of whether a message will be spam

I couldn't agree with this more. I want people to remember this whenever the topic of decentralization or federation comes up. People see this as a technical problem. it's not. It's a political and organizational problem. Even with email, which is fully decentralized (other than the ICANN TLDs) running your own node still incredibly difficult. And those reasons aren't technical at all.