To my mind, email is a 100% decentralized system that we have today. This, to my mind, suggests that we fully capable of designing, implementing, and widely deploying distributed systems. Can you help me understand where I have erred in this logic?
If a decentralized solution is not easy to pick up, and does not offer clear everyday benefits to users over existing centralized solutions, then it will not win with users. (Just "better privacy" is not really a good everyday benefit for the average person; compare DDG usage to Google.)
If you choose to keep going down the rabbit hole, please learn first about how email works.
Email is centralized in the exact same way Facebook, Twitter, or Netflix are centralized--probably more so. There is less CDN, etc. infrastructure built for email unless you are one of the really big players.
There isn't a large infrastructure set up for email because it's so damn efficient that a 1990s era computer can serve as an email server for thousands of people.
Every organization can host its own email programs: servers, clients, spam filters, etc. Most businesses and companies still host their own email.
Do you have a source for this? Most companies I know of offload mail hosting to gsuite or Microsoft.
But yes, you're right, the income is absolutely centralized.
[1] https://hashbase.io