Dilution will always happen with user submission sites as the number of users grow, specially if there is no moderation.

Digg/Reddit,they all started as some niche tech site and eventually turned into political/media/news sites (atleast digg was, reddit's savior is its subreddit feature) once the userbase increased.

The only way to keep HN niche is by introducing categories and let people subscribe/unsubscribe to those categories or through strict moderation.