> It's also only decentralized as much as public email infrastructure is decentralized.

So… entirely? What am I missing about your point?

I run my own email servers, but 99% of mail goes over Google/Microsoft/AWS/etc email servers anyway.

In practice, it's quite centralized and you're always at risk of one of the big providers locking your servers out of their network or putting you on a blocklist they all use.

Public email infrastructure is almost entirely dominated by Google. This is worth looking into if you’re not familiar with the state of affairs