But, email is only decentralized in theory. In reality, email was fully captured a decade ago by a monopoly of 2 particular inbox providers.
In consumer, Google has a defacto monopoly and runs the show. In B2B, Microsoft has a defacto monopoly and runs the show.
Nothing can change without Google or Microsoft making the move first. And neither of these companies have any interest in changing/improving anything, given they already have a monopoly in their respective corner of the market.
What we need first, is to lower switching costs to open up the market again. This could mean making DNS less of a nightmare so domain-based email becomes easy again. This could mean a government mandate that your email address (like a phone number) must be allowed easy transfer to other providers (Since Google owns the Gmail.com domain, they own your "phone number" in a sense). Etc. Etc.
Imagine if Google and Microsoft owned your physical mailbox...and they decided what type of letters you could receive from who...and they sold ad-space in it. That's essentially what we've done with our digital mailboxes.