I don’t see how you can make this common. People will centralize around large inbox relays. This always happens. Unless the inbox relay is the phone/desktop node itself over tor or something, and clients opportunistically keep retrying the send, but that would be unreliable.

Make deploying a DM inbox server as easy as creating a new spreadsheet on Google.

I completely agree with your point of view, but popularizing it among ordinary people who are not professionals requires simple and easy-to-understand language.