1) Design decentralized service, with low barriers to entry
2) Jerks start using the service for bad things that make life miserable for the rest of the users.
3) To combat the spam, some service providers start creating whitelists of which other service providers they will talk to
4) People flock to those service providers, because they are the only ones that aren't flooded with spam.
5) Holdouts end up HAVING to switch to one of those providers because they are the only ones anyone talks to.
6) The service is no longer decentralized in practice
I don't know how we get around this problem. These people complaining about not being able to send cryptocurrency emails through mailchimp are complaining about the centralization of the service, but most end users are thinking, "I am really glad I dont have to deal with the spam they are trying to send me."
Most people WANT some barriers to entry for services, because without them, spammers will always win.