Aside on self-hosting: This reminds me a lot of the self-driving car accident debate. Self-driving cars could be thousands of times more reliable than humans, but unless they are mathematically perfect, some people feel safer driving themselves because they feel they are in control. Nevermind that Google/Facebook etc have millions of man hours invested into security.
In essence, it's an economic argument, not a technical one.
I don't see how this is true at all. Most people don't care/know about security. See Wordpress. Many companies don't take security seriously, there are many open mongodb pointing to the outside world - HackingTeam, a team chockfull of blackhat hackers, was partially done in by sloppy authentication and passwordless databases.
Today, when people run their own server, it doesn't seem the incentives are properly aligned at all - and given that even blackhat shops don't even take the time to secure their own systems properly, the economic argument falls flat.
It's been tried, again, and again, and again, and it fails for the same reason all such projects fail - the only people who care about this architecture stuff are those who are geeky enough to be running their own decentralized services.
Heading off the inevitable response: Email is not a social interaction service, it is a message passing service that happens to have social uses.