When it came to email, we didn’t care about the system becoming too centralized. They were all run by large companies that had the resources to keep things running for a long time, so no need to consider how long your email will survive. Most people didn’t even know they had options. If someone you know made an aol email, then you also made an aol email. The content you receive is exactly the same regardless of where you sign up. No need to think about who federates with who, or how the instance gets moderated. Email is email.