With Delta Chat nowadays you can easily host your own federated and secure chat service. It is great.

My idea is basically person to person comms. Why can't they just send messages directly to each other? I guess mobile can't do this and relies on polling (i.e. you can't expose a service running on the phone for security reasons). And when the group gets large enough, beyond say 3-4, sending messages to each recipient gets unwieldly

> Mails are superior in announcing to multiple people

People who are known at time of sending. A slack message can be searched by those joining the team much (much) later, those who move teams, in-house search bots, etc. Mailing lists bridge this gap to some extent, but then you're really not just using email, you're using some kind of external collaboration service. Which undermines the point of "just email".

There are technical advantages to a dumb client. The more you outfit an XMPP server with basic things modern users want like message history and push notifications, the more state and responsibility moves to the server.

Disclaimer: I have a lot of XMPP experience but have never used Matrix