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.