It is so very common for projects to inflate with new features until they become useless piles of filth. The fact that HN is a bit clunky is a feature not a bug. It will only attract and retain a certain type.
I get that people don't want HN to get worse than it is. But i don't get why people think open sourcing it would result in that.
I trust PG would be the kind of project lead who only accepts pull requests that would actually improve the site.
And I trust the community has a lot of smart people who would come up with solutions that would actually improve this site.