I would venture a guess that even the most technically savvy users who could set up their own mailsever now, still flock to "Big Mailer Corps" for the easy to use mail client filled with useful AI features, etc. and that even in the presence of an easy alternative to set up their own, will end up forwarding to their "Big Mailer Corp" of choice that offers them the most efficient user experience for their workflow.

IMHO, the most important thing missing right now to promote more decentralzied mail is a cometivtive open source mail (web) app, with a simple UI and a vibrant plugin system for every use case.