On the side of centralization is a market worth unimagined billions, and perhaps seven providers that already routinely lobby to get what they want.
On the side of decentralization? Little guys who are already being bullied and don't have the resources. Politicians who have other extremely dangerous problems currently, including and especially those in their own industry. Don't expect their help for a long time. Smaller service providers who only want the chance to switch sides. And large independent organizations (businesses, schools, governments) who are really only interested in a cheaper bill for the same service quality.
Look, we have struggled to fix the MITM/plain text problem and spam/origin problem for well over 20 years, and the Internet Age is maybe 30? It's extremely likely that the only way to fix the core design problems with email is to have the service consolidate to a small consortium of services, have them improve the base design, and then fracture the monopolies. That's going to take decades.
And mail is still such a pain in the butt for a user document database. And the user experience is a joke! Asynchronous delivery, best effort failure? I mean, what? The average web page is orders of magnitude larger now! People complain about FAX and FTP.
A new, chat-styled mail protocol can be designed to address all the problems above while reduce the risk of abuse. I wonder why nobody standing up to do it.
Unfortunately, we are back to the age old question. How can we have an ecosystem where anybody can email anybody without it being taken over by spam?