That could be worked around, contributing could be invite only, but also allow a non-invited contributor to post but their post would be collapsed (or at a negative score) by default until an invited contributor voted it up.
There will be pros and cons to every system. How else do you think we can decrease the growth rate of the HN userbase, which is probably the root cause of the decline in quality comments and threads
Won't it make it just an echo-chamber that would increase group-think? I like the idea that new people can join and contribute or learn from others without having to first curry favor to get invites.
That will certainly ensure only the acceleration of the decline. The ecosystem will be isolated and perpetuation of 'preaching to the choir' will ensue.
How would have all of the now-members of both HN and the greater 'community' have stumbled upon this resource if it was invite-only?