While I think slack is much more useful that any of the ones you mentioned due to 'unique' implementation but it's not hard to replicate. Plenty of communities

While I think slack is much more useful that any of the ones you mentioned due to 'unique' implementation but it's not hard to replicate. Plenty of communities already use free alternatives like Discord (despite it's focus on 'gaming' communities). I expect open source solutions (Matrix mostly), both self hosted and ran by low cost hosting providers, to start taking Slacks market share once they reach feature parity & a level of polish.