The best way to optimize HN's value to YC is simply by having it be as interesting as possible. Curiosity is the reason the community comes here, and the community coming here is why HN is valuable, so to do it any other way would trade a global optimum for a local one—basically trading the goose that lays golden eggs for some eggs. We're in an unusually fortunate position this way. If HN were a startup constantly needing to get its numbers up, we'd be under business pressure to do things that made HN less interesting. It's a happy coincidence that the most-interesting HN is also the HN that most serves YC's long-term interests.
Secondarily, HN has job ads and (more recently) 'Launch HNs' for startups that get special treatment. The job ads are written about in the FAQ and the launches eventually will be, but we're still figuring out the best way to do them. These are concessions HN grants to YC in return for being funded by YC, and our intent is to have them be both secondary and obvious. I agree that it's fair for HN to make these concessions and especially with your reason for saying so: it's the way to have a sustainably high-quality site, which does not come free. What HN doesn't do is make non-transparent concessions, like moderating anti-YC stories differently. But I've covered that elsewhere in the thread.
Secondarily, HN has job ads and (more recently) 'Launch HNs' for startups that get special treatment. The job ads are written about in the FAQ and the launches eventually will be, but we're still figuring out the best way to do them. These are concessions HN grants to YC in return for being funded by YC, and our intent is to have them be both secondary and obvious. I agree that it's fair for HN to make these concessions and especially with your reason for saying so: it's the way to have a sustainably high-quality site, which does not come free. What HN doesn't do is make non-transparent concessions, like moderating anti-YC stories differently. But I've covered that elsewhere in the thread.