Plenty of organizations and individuals run their own servers and have no problem exchanging email with Google, Microsoft, and other large providers.
The point is that they can and do because the system is not, in fact, centralized.
Also, judged only on the volume of spam and abuse, many systems besides email -- decentralized and centralized alike -- could be called a "total failure." And yet somehow these systems remain stable and functional and useful.
The only problem with doing this could be mail deliverability. But if you setup SPF/DKIM/DMARC, and use a mail service, it works fine if you're not spamming. Mailgun, Amazon SES, Mxroute, they'll all deliver your mail and for cheap. There's no single point of failure needed to get your mail delivered.
For some definitions of fine, sure. I run my own mail, and relay it through an AWS EC2 instance that I've used on the same IP for over 5 years. It took an exceptional effort over many months to get off the Hotmail blacklist a while back. I still wonder when emailing someone I've never emailed before if that message will go do a Junk folder, or be delivered.
The benefits outweigh the costs, but it is not easy, and you can start having problems that are very hard to solve.
You don't need to use a delivery service. It's true you need to set up SPF/DKIM/DMARC for many peers to accept mail from you, but then you can deliver the mail yourself directly.
Yes, this is true, but I still get better delivery through delivery services. For example a lot of email providers send any email coming from a cloud hosting IP directly to spam.
Even with that most big providers (looking at Outlook) still use IP reputation. Some popular email blacklist providers will list entire /24s if they receive enough abuse complaints so you generally need a high quality provider that takes abuse seriously to prevent getting caught up with a bad neighbor or your own IP block.
After using the same Digital Ocean IP for 8 years, I got unlucky and their /24 block got listed on spamhaus (or mailcop, or similar) and major providers (Outlook and Gmail) stopped accepting messages from my server.
The point is that they can and do because the system is not, in fact, centralized.
Also, judged only on the volume of spam and abuse, many systems besides email -- decentralized and centralized alike -- could be called a "total failure." And yet somehow these systems remain stable and functional and useful.