It really is amazing, I've been here nearly 12 years now and the quality of discussion hasn't devolved. If anything, it was a bit more cynical/negative ten years ago than it is now.
Your comment gave me an interesting idea. What would happen if we imposed a minimum char-limit for say, hacker news comments? On one hand I'd expect more descriptive responses, fewer comments that are a short "Your wrong but I won't tell you why", etc. Comments just spamming letters would be fairly easy to detect.
On the other hand what would this minimum char/word length be? Even your comment is fairly short, but it's descriptive and fully communicates your point...
Huh. I've only been here a couple of years, and I feel the quality of discussion is already starting to worsen. (However, it's still better than most other things—see https://danluu.com/hn-comments/)