What you're describing is redundancy, which email also has. Not decentralization.
Discussion
Mmmm... no, it's not the same.
I can send an email from different SMTP servers, but messages not coming from the "right" SMTP are nowadays mostly discarded.
And even during the times when that used to worked the recipient couldn't verify its authenticity.
And even if that really worked today then I won't be able to receive replies unless I submit myself to centralization (single point of failure, the postmaster.)
See? You don't understand how it works.