If it were a question of letting "passionate, smart people have it out", then sure. Unfortunately that's not what you get in practice on the open internet. What you get are (a) people who want to vent their personal issues in the form of public issues, and (b) windbags with a lot to say, who think themselves smarter than they are.

These types can quickly dominate the threads. That repels the smarter people who have no interest in such threads. This is how HN could fall into a death spiral. Worse, the risk grows as the site grows, because then more people are attracted to it for wrong reasons. (Right reason: curiosity. Wrong reason: hearing the sound of one's own voice, or making others do it.)

I think the only way you could sustainably have a forum of the kind you describe would be to close it off from the public internet. We're not going to do that with Hacker News. This arguably does leave the site "a little flat"—I've made the same point in the past using the word bland instead of flat. But better flat than dead.