https://rocket.chat/
https://zulipchat.com/
https://matrix.org/
https://mattermost.com/And honestly, of all the alternatives, IRC is by far the most popular one (and the other alternatives have IRC bridges) hence the most likely to stay around the longest.
Also the simplest to implement, which helps a lot too.
And I'm personally working on Quasseldroid, improving its UI to make it much more usable.
If you have suggestions on how to make Quasseldroid usable for the people that refuse to use IRC, I'd love to hear them :)
Sure, but the reason all the developers at my workplace use Slack is because we have to communicate with a lot of people who aren't developers.
Slack's good UX extends beyond the basic chat interface to configuration, administration, and initial signup. It's significantly easier for someone nontechnical to toss money at Slack to get a new private space than it is with IRC. The first page of "IRC hosting" search results on Google for me are mostly shell accounts; nothing turnkey and professional looking that a business person could/would use.