Realistically unless you have someone who is willing to spend 1,000+ hours per year making qualitative judgments about the quality of every active contributor, and who is then willing to go around being kind of an asshole, the community is eventually going to fall apart. And right now that's not going to happen, because there's no profit motive.

You could do something where you collect everyone's email address, and then give anyone willing to spend the time to be HN's curator the ability to send out 3 emails a year to the entire list that they could sell advertising against and promote their own projects. But short of that, I don't see anyone stepping up to put in the time and emotional labor required. The only other solution is to implement a paywall, but I don't see that happening any time soon.

Yeah, a one-time paywall is what has kept Metafilter and Something Awful going strong. Don't think pg et al. will ever pull that though.

I think you may have mis-assessed "no profit motive"

HN support YCombinator in several ways. In particular consider how much less visibility YC would have without HN.

YC has been very clear that they evalute HN contributions of new applicants.

I am not being critical of YC or HN, I just wanted to point out that YC is run as a money making venture and HN is managed to support it.

"HN support Y Combinator in several ways."

The maintenance that HN receives is proportional to its value to YC. This is a problem, because as the userbase increases, the maintenance required and the marginal value created scale at different rates. It's not a show stopper by any means, but eventually there may have to be some sort of realignment.