I run a group of 20,000+ people, and if I didn't set the tone on what the group is about, regularly, the value of it would drop to nothing.
I love my community. But I absolutely recognize the role I play in keeping it a nice place to visit.
Enough HN members are happy to feed the trolls (even if unwittingly) and encourage more without removing them.
Downvoting, flagging works even when the mods are not around. I am pretty sure that a comment flagged too many times is greyed out and almost invisible (and that happens without mods).
Again, if we are to believe there are only 2 full time moderators on HN for the amount of comments going every single minute, it's virtually impossible to rely on mods alone for proper discussions: simple maths.