There's a huge difference between running your own mail server and running a mail client. Most people are not well positioned to run their own mail server.
People werent well positioned to run their own web browser either.
We can make it happen, if this new thing is so attractive people would will do it, just like they put up with Windows for several decades now.
It could be a simple raspberry pi box that has all the stuff ready-made, just enter wifi credentials and whatever and run. The box has to be in white and silver because then who wouldnt want one? To show it off like a status symbol. A sleek little box in a corner, "my own email" people could say.
Mailpile is an MUA, not an MTA.