Leaving free, open source software for proprietary alternatives I can see the end of FOSS and GNU/Linux at the end of this way. Why Slack and not Matrix for example? Slack doesn’t have a desktop app for Linux even. Only for Mac and Windows!
I use Linux, because of the Free Software / Open Source character and I think software around such community projects should also met such conditions. So in my opinion Slack is not an alternative for IRC, because it don’t met such conditions. I can understand, that you think IRC is outdated, but then please find an free open source alternative to it.