With Discord if you want premium stuff you pay for it at an individual level, and having the free tier doesn’t limit important functionality like message history.

With Slack your workspace’s cost is based on how many people are members in the workspace. You can’t have one member decide to buy a premium subscription and another one decide not to. You’re either all in on paying money for every user or you’re stuck on a gimped free version that hides message history and has other limitations.

As someone who experienced online chat rooms, forums and IRC....it is crazy to me that something like Slack is even successful.

And there's a bunch of neat features that are somewhat unique, like using roles to sign up for specific types of announcements. It can be a chat room, or a forum, or voice/video chat, or a substitute for email notifications, or all of the above. I think that versatility is important too.