to me seems like a valid critique of self-hosting which I largely agree with (not everyone will run their own email server, and being a lone sysadmin is kind of an anti-pattern), but there are also other responses to this besides embracing the status quo, as you also suggest:

• coop-cloud (not self-hosting, but hosting by your local community collective of choice)
• peachcloud, yunohost etc. (make self-hosting graphical and easy, can also be combined with first point)
• p2p systems that have less burden of hosting

p.s. thanks for sharing and appreciate your framing of ease-of-onboarding as a research topic