heck if anything, the Fediverse, as it is right now, is more susceptible to this. You can spin up spam instances on new domains, with spam users, and have them federate to existing instances, faster than volunteer run instances can ban/defederate.

So you end up with not federating by default, but having some trusted web of instances that federate and maybe an approval process for new instances to federate with? But that’ll still lead to centralization with “trusted” instances and new instances having a hard time to join the club (there’s also the scaling problem of the fediverse, but that’s besides the point). So you end up with a few very big instances and the owners of those instances having all the power. Or maybe small isolated islands of mutually trusting instances? Still better than tech oligopoly, but also a far cry from the original dream.