To answer how it got so big: it didn't start out trying to replace Slack. It just solved an acute pain point for gamers. Skype was becoming increasingly enshittified, and people were floating between TeamSpeak, Ventrilo, and Mumble, none of which were that great. Discord captured the market because it was completely free and had the audio mechanisms in place to make people with shitty mics and background noise tolerable without forcing everyone to use push-to-talk. That’s really it. By the time non-gaming communities were looking for a Slack alternative, they just defaulted to Discord because 90% of their target audience already had the client running in the background.