Every time this has been tried, it's failed miserably, as it should. People need to be able to post anonymously. Otherwise it's too easy to target the messenger, rather than the message. Think journalists, hackers, abuse victims, political dissidents, whistleblowers. Yes, this makes moderation harder. Sorry but that's life.
The vast vast majority of people are not those things. Even if you wanted to be anonymous to other people you could still prove your identity to a service and the service could keep that identity hidden.
Part of the strength of your anonymity is based on the cost for anyone trying to figure you out. Centralizing your trust in a service that promises to keep your identity hidden is begging for trouble.
The service doesn't have to be malicious and can genuinely make every effort to keep your identity hidden. Trusting services like that creates choke points where it may not have been viable to attack the service for one identity but for hundreds its worth it.
Theres no such thing as flawless anonymity and you can't escape having some number of trusted parties, but you want to keep that list as low as possible.