Thing is, Discord is a chatroom, and chat and forums are fundamentally different. Realtime vs. slightly less constant. Forums produce less of a flood. More easily searchable, allowing quote replies (something Slack has Discord doesn't). Forums might be old-school, but I find them eminently more readable than commercialized chatrooms like Discord or Slack. And they allow for more powerful discussions than quasi-BBS like Reddit or Hacker News.
You could theme your phpBB forum, install mods, add lots of fun flourishing touches (like custom ranks), etc. Some forums displayed avatars and post counts, others didn't . All that made every single phpBB forum feel like its own distinct community. I don't see that with Instagram.