HN is no more or less pretty than Reddit, and the general feeling around here is that Reddit's Eternal September happened sometime in the last year or two.
HN won't last forever. Reddit was great when it started, then they achieved the growth they were looking for and it got diluted a bit. PG started what would become HN, and the early adopters migrated. As reddit becomes less useful for tech and programming news, or as a way to find cool new sites, more people will migrate over here. Then some people will get pissed at the requisite drop in quality, and will start a new place.
All this has happened before, and all of it will happen again.
You are all acting as if the fate of news.yc is in your hands. It isn't!
If anyone here proves that he is not a good actor, he will be ruthlessly pruned from the site. This happens to dozens of people every week. This is all handled behind the scenes by pg and the editors.
That's the real reason this site has lasted as long as it has: pg and co are willing to be extremely ruthless about weeding out bad actors. That's the big difference. Other sites were more interested in growth, so they weren't as free to get rid of badly-behaved members.
This idea that news.yc is doomed to follow in the path of all other discussion sites that came before it: no, it is not. Not so long as pg is willing to keep pruning bad actors.
The flaw in your expectation is your assumption that communities decline due to bad actors. I think you're wrong. The decline comes from two other effects: regression to the mean, and decreasing overlap in shared interests and goals.
People who followed PG's essays from the early days were pretty plugged in to tech startup culture, and the early community was focused around that. They were people who had a startup, or were contemplating doing one. Generally, they were smart, ambitious and technically astute.
Over time, those traits regress to the mean, as people come upon the site based on more and more random hearsay, wade in and contribute. If they are marginally less smart or astute, they still aren't bad actors; but the group's average declines, and asymptotically approaches the population mean.
And this more diverse population isn't as homogeneous. That affects the kinds of stories and comments that get voted up. It's only natural that stories and comments which appeal to the lowest common denominator will attract the most votes; it's that in the past, the lowest common denominator had a higher value, owing to the homogeneity.
HN won't last forever. Reddit was great when it started, then they achieved the growth they were looking for and it got diluted a bit. PG started what would become HN, and the early adopters migrated. As reddit becomes less useful for tech and programming news, or as a way to find cool new sites, more people will migrate over here. Then some people will get pissed at the requisite drop in quality, and will start a new place.
All this has happened before, and all of it will happen again.