@evan: I don't know how to avoid the centralization that plagues email, but I think a crucial element is for the Free Software community to develop tools to make it easy to run an instance without sysadmin experience.
We can't hope to reach the convenience of something that someone else maintains (i.e. "register and then use blissfully unaware for the next 20 years"), but we need to aim to get as close as possible.
Currently we have tolerable answers for "setup your own instance", but we need to deal with hardware failures, DoS attacks, regular upgrades, moving on to a new software when the old one is discontinued, managing quotas, new registrations, payment, ...
FreedomBox is a good step in that direction, but it still mostly focuses on setup and upgrades, with no good answers for how to deal with the nastier parts: it's easy to find some free time to get started, but you don't get to choose when hardware fails and your users will get angry quickly if doesn't get fixed *very* soon.