Let's say there never was an SC forum and all the same people from SC actually met in a random toxic discord server. Well there's thousands of discord servers full of oddball characters and trolls, so it would be nothing special. It would probably end up dissipating at some point due to some shitty mod and leave barely an imprint on its user's memory.
Compare that to the SC forum. For better or for worse there was something about it that kept people around. Maybe it was the unified theme, or the layout, or the way the threads and the chat are both very easily accessible... But something kept people around. I really couldn't see the same happening if this was just a discord server.
There's something kind of cheap and gross about discord and the culture of many of the servers. Sure you have all these features and commands and bots and fancy stuff, but it reeks of NEET energy and try hard edgy desperation. I can't necessarily say the same of the forum though it often can be the case here too.