Back in 2016 I came across a computer science paper where the author had studied the decentralization vs. centralization phenomenon in various cases (not just in IT but also in real-life social networks, economic dependency graphs etc.) and concluded that every man-made decentralized system will eventually exhibit centralization to some degree as it will develop nodes that have orders of magnitude more edges than the average node. It seems to be a very natural phenomenon. I wish I remembered the author or the title…
As another commenter has pointed out, Delta.chat shows just how closely an email client can mimic the chat/IM interface.
At least an email to the user's @Facebook email address gets picked up in messenger.