Discord if you don't mind something proprietary, Mattermost or Rocketchat if you do, Zulip if you want something slightly different . . . and no doubt many other alternatives

Slack is easy to replace with something cheaper and better on a product or technical level. The network effects are strong of course, but they won't sustain it forever

Discord is a solid product. They just need to launch a simple business-friendly alternative UI without the teenager gamer aesthetics. I’m surprised they never tried going after the enterprise market.