I suspect that the HN software does much more important but less obvious things which meet YC's goals than those you mention. I also suspect it does those things well.

As for page expirations, well, they aren't a deal breaker for me.

If I was PG I would test YC founder's time/frequency spent on HN against the success of their startups. I would use that data in evaluating new applicants. This test might ferret out founders with a procrastination problem.

I might also try discovering possible throw away HN accounts created by founder's applying under their official HN identity. That could really ferret out some assholes not technically competent enough to cover their tracks and dumb enough not to think this possibility through.

Also, PG's essays are one of the great success stories of content marketing.

That's how I found this community, and although I probably will never apply to YC, the enthusiasm for it's backed startups have rubbed off on me. I wouldn't have been an early AirBNB or Dropbox user if not for those essays.