Email is decentralized to us. We have the very specific skils, attitudes, and patience required to buy a domain name, rent a VPS or dedicated server and configure SSH public keys, Postfix/Courier, DNS, SPF, DKIM, and keep it up to date and patched, audit it to make sure it stays secure, and detect/respond to intrusions. And even then, unless you are going to roll your own emergency power, multiple redundant HVAC, and fire supression, as well as shell out hundreds of dollars a month for significant upload bandwidth to your house or apartment, your data and encryption keys are still in the physical possession of a corporate datacenter.
Email is most certainly not decentralized to anyone outside the IT world. They can, with extreme difficulty, move between providers, but they are still dependent on a provider.
Email decreases the volume of users that mediocre players can own (Google does email better than anyone else, so it gets a significant share of the market) but users are still going to be owned by someone.