The only way to truly guarantee it would remain high quality would require credentials to use the site, or require invite/referrals, but then that has a whole host of its own problems.
I'm relatively new so I don't know what HN "used" to be like, but in the short time I've been here I've noticed it decline. It seems to me that more and more people who aren't knowledgable or have insights to offer are joining and people like jacquesm and riderofgiraffes are leaving. It was inevitable and has happened in every community I've ever used.