Matrix is distributed chat system that is free software and people really should be switching to rather than everyone moving to Slack.

IRC and mailing lists have worked for 20? 30? years and continue to work today.

Well, then we can all turn off our computers and go home, because something that "works" is obviously good enough and the entire software industry doesn't need to exist anymore. Most things we have today "work" but we're still improving on them because "good enough" isn't good enough.

The question you're not asking is whether your replacement is better than the existing solutions. Slack isn't. It's closed source, proprietary, and tied to a third party's servers. I'm not going to tie my projects or company to that kind of anchor.

Absolutely. I completely agree that Slack brings too many problems to the table to be a replacement for IRC+Mailing lists.

I'm just saying that IRC+Mailing lists aren't good enough.

Exactly, that’s why we evolve them (see ircv3.net), or try to reinvent them (see matrix.org), and don’t try to make a proprietary clone and sell it.

I might be wrong in saying this, but I suppose that most open source projects use IRC, and the larger, important ones use mailing lists.