I don't really see a lot of overlap between these technologies. To me, forums are useful for getting help / sharing knowledge on a particular topic, reporting bugs / checking for known issues in an application or product... Things like that, where the organization and retention of the information is a benefit.
Discord is a place for keeping up with friends, finding a group for a game, or discussing something current with people that share an interest (e.g. discussing the latest episode of X show). Slack is for keeping up with current things and chatting with team members at work, and following alerts for an application that you're supporting (because that's way better than email alerts). I recognize that there are people that use these technologies differently, but they each have their own niche that I wouldn't want to use the others for. Forums are not a great tool for instant communication or relatively "chaotic" discussion (it's a lot harder to follow the splitting chains of thought compared to breaking side conversations into threads that are still easy to follow along in a channel), and nobody wants to constantly refresh to keep up with the conversation.