I don't understand how "multi-channel IRC with history, multimedia and good UI/UX for the desktop" is such a small market with so few competitors while "single-channel IRC with history, multimedia and good UI/UX for mobile" is such a saturated market. For the latter you have WhatsApp, Instagram (yes, really, IG is the main communication app for my generation in my country), SnapChat, Telegram, Signal, Threema, Sessio…
I wish Mattermost wasn't always trying to nudge you out of community version, but otherwise pretty solid, better than Slack IMO. Is unfortunate they require weird gitlab spoof bypass to use SSO in community version. Shameful it's not out of the box.
Many years ago Pidgin with multi-channel IRC was all I needed, but seems Slack killed that whole party, which brings us to the current situation :(.