Even if you were consider only the benefits YC gets from HN (exposure for YC companies, recruiting, and cachet), the benefits to the first from larger crowds are, I'd bet, offset by the slow dispersion of high-value members that has accompanied the drop in quality we've seen over the last couple years.
I'd be willing to bet that HN would probably serve YC's purposes better with 5,000 - 10,000 users than with 50,000 - 100,000.
I think craigslist is a great example of success going with the most minimal "innovation" - in the last ten years I can think of two changes there, and one wasn't even voluntary. First, the map-view for housing listings and second the giving up of sex services to other sites like backpage under threat from a bunch of point-scoring district attorneys.
Just because you have grown in the past with a certain tactic does not make that tactic the most effective choice. And, certainly, if the industry changes so much that mobile does indeed take over the personal device market, having a subpar experience on mobile phones could then have an effect that previously wasn't apparent.
"We make the best candles in all of the world!" didn't allow the candle company to compete with the lightbulb maker. (Albeit a less than accurate comparison, it illustrates my point I think.)
Video/Audio/PDF Presentation:
http://www.lukew.com/presos/preso.asp?26
If you want a mobile first version of Hacker News for now, a fan created a nice HTML5 app version of the site.