HN can(which is in question) maintain a relative high quality discussion towards a focused and limited subset of topics (tech, science and certain social issue

HN can(which is in question) maintain a relative high quality discussion towards a focused and limited subset of topics (tech, science and certain social issue like surveillance) is because it is a niche forum by default. It is HACKER NEWS, rather than popular news. Reddit is just too board. However, certain subs are still pretty interesting and informational, until....it becomes popular. So yes. Popular means low quality. It is sad, but inevitable..