While I ostensibly agree about learning how to deal constructively with opposing perspectives, I also don't think online discourse (main stream avenues) will ever be the place to learn or partake in those sorts of conversations. Even in smaller subreddits, your comments will be viewed by thousands of people, some of whom are explicitly there to troll or to argue in bad faith or even people literally having mental breakdowns. You also end up in situations where every reply is to a new person, so you're not really having a discourse with anyone just an amorphous entity. Look at things like "Godwin's law" or "Poe's law", for some long running beliefs/commentary on internet discourse.