That’s not the reason email has re-centralized. It’s because big-tech companies provide it for free in order to bind users to their services, and at least in the case of Google, also to collect data and drive ads.
As someone who administrates their own email server and works at an SMB who administrates their own email servers, it works quite well and doesn’t require centralization.
"When mice march with elephants it's in the mices best interest to be cautious"
I've done a lot of a lot of email administration over the years, servers with 100,000s of accounts, and you walk a very fine line in being able to communicate with the rest of the world. Your IPs must exist in certain blocks (or at least not be in banned blocks). You must block outgoing spam messages at a much higher rate and quality than google/hotmail do. Get yourself blacklisted for any reason and expect to disappear from the internet, meanwhile no one is going to block the largest services.
As someone who administrates their own email server and works at an SMB who administrates their own email servers, it works quite well and doesn’t require centralization.