"Polluting HN" is more than just the vitriol thats flagged - there are plenty of self-promoting, downright wrong, or comments clearly unrelated to the posted article but comment author gets to segue to their hobby-horse (off-topic discussions are annoyingly frequent, IMO). HN is better than most, but its not immune to being gamed. Once there is a financial incentive for it, it will become more common - see how Twitter turned out after offering monetary incentives for engagements.