I ran my own email for close to 15 years, most of that time on the same IP address and the same old domain. I could never get GMail to reliably accept my mail, and of course there is no support to find out why. I set up and tested their documented best practices, and had no issues corresponding with a lot of people.
Spam is not an issue in practice. Two basic filters drop 95% of spam without scanning the content, and an open source reputation+content engine drops the rest to the tune of maybe 1 spam a day. The two annoying parts are dealing with exceptions, often large companies misconfiguring their outgoing email; and the open-source nature of the filtering engine’s ruleset, meaning it occasionally completely breaks for a short while.
Wouldn't that be the "best case" for email because of the centralized idea? In reality it's a few huge companies that matter and if you end up on one of their lists because of some bad IP, missing header etc. you are a bit out of luck.
We now have several new social networks based on the ideas of open protocols and federation. They are not yet large enough to attract too much attention but we’ll see if the story rhymes.