I'll just comment on IRC since I know nothing about the others.
As much as I'd love to say otherwise, IRC is not a viable Discord alternative.
The onboarding on Discord is as smooth as IRC would be if the standard web clients had actually decent UX.
Unlike Discord, however, people can't seem to figure out banning on IRC.
There's no realistic way to CAPTCHA on IRC connection.
You could work around that on very modern IRCds with something like requiring SASL for authentication and having a web-based registration that does CAPTCHA, but mandatory registration breaks the onboarding process again.
People these days have a hard expectation that they can just drag and drop files and have it work, rather than navigating to imgur/some other site.
They want server-side logs to catch up on conversations they weren't online for -- rather than setting up a bouncer (which I'd argue is a privacy nightmare rather than a feature, but YMMV if you've got nothing to hype).
Some even want voice chat and avatars.
In some rare cases, people even have to call their ISP about unblocking 6667/6697 because of old cases with botnets.
Web applications would be much more popular, but then other IRC people want ways to ban the web clients reliably.
And let's not get into the historical cruft of the ident field that's not entirely historical cruft because it's actually used by shared providers to enable banning in the first place.
But that requires, again, knowledge of how to ban properly.
Which is a lost skill.
tl;dr: Discord UX is so much better than IRC UX to the a lot of people in a lot of ways.
I haven't checked back in a while but last I saw, Matrix looked a lot like glorified IRC with extra stuff tacked on.
And I would also require that people could join over links with similar capabilities like Discord (require Account Age, registered with Telephone numbers, automatic joining into specific room, room visibility and access based on roles, prevent embedding of media based on roles, proxying of any previews in the chat via the server... etc)
Because that is what some people want from a chat service. They want to be able to scrollback and see what happened overnight without operating some kind of server for that. They also don't want to necessarily have to login or have to worry about private keys.
Why is this still not a solved problem ? Why is there no free open source Skype alternatives !?