Reposting this in comment from a reply elsewhere in the thread.
If anything there should be SOME centralization that allows other (known, somehow verified) instances to vote to disallow spammy instances from federating. In some way that couldn’t be abused. This may lead to a fork down the road (think BTC vs BCH) due to community disagreements but I don’t really see any other way this doesn’t become an absolute spamfest. As it stands now one server admin could spamfest their own server with their own spam, and once it starts federating EVERYONE gets flooded. This also easily creates a DoS of the system.
Asking instance admins to require CAPTCHA or whatever to defeat spam doesn’t work when the instance admins are the ones creating spam servers to spam the federation.
That’s happened with almost every VPN provider that has claimed to be no-log and then got a government subpoena. At that point you have 2 options: A.) Shut down, or B.) Code a technical way to capture the requested information for that user.
Sometimes they do choose to shut down and sometimes there isn’t a technically feasible way to get the information.