You are seeing the quality go down because YC requires applicants to join HN so now you have thousands of applicants who would have never joined if YC didn't required it. I believe this plays into the quality going down but I could be wrong.
Perhaps a clarification from anyone downvoting you as far as why you are being downvoted which I assume is because of this statement "quality go down because YC requires applicants to join HN"?
Or is it "now you have thousands of applicants who would have never joined if YC didn't required it." (sic)
For clarification, per PG:
"The first thing I notice when I look at an application is the username it was submitted under. If it's one I recognize for making thoughtful comments on Hacker News, I give the application extra attention."
So I can fully understand that while joining HN is not a dejure requirement to getting accepted to YC it certainly seems like a defacto requirement based on the above statement.
No one's complaining "too many usernames are taken"; people are complaining about the comment quality.
When you get a HN username to make a YC application, you're going to be hyperaware that you're here visiting pg's home turf. You'll either try really hard to post high-content, insightful comments, or you'll simply lurk. Either way, you're not likely to be wandering around urinating on the furniture.
tl;dr -- the requirement seems more likely to boost, rather than harm, comment quality - if it has any effect at all.
