There has been no decline in quality and blaming cortex is utter bullshit. My guess is that the haters never liked mefi in the first place and are abusing the change in management to attack the site.
But there are people commenting who say they have visited the site for a decade and got bored of it when it became an echo chamber.
Perhaps all sites like it decline. Slashdot, Kuro5hin and many others are a sad shadow of what they used to be.
Maybe HN in 5-10 years will be pretty sad too.
It's finally dead!
I think that people's memories are flawed enough that the "it used to be so great" arguments are all pretty suspect. I tend to think that it's more like "I used to not be familiar enough to realize x" combined with "I feel like I've seen these arguments before". I have the same feelings about HN plenty of the time.
The other big thing which is hard to under-estimate is how much the web has homogenized over the years. MetaFilter was where you went to find someone's personal blog, collection of cat scans, etc. and people just don't do that the way they used to: instead of blogs that largely happens on Facebook or Instagram (and it's private), individuals are competing with for-profit content mills and SEO spammers, etc.
HN has followed a similar arc – you used to see more personal tech blogs and open source but now that's competing for attention with major companies paying staff to blog as a recruiting effort. Nothing wrong with that but it feels rather different and it changes the values of aggregator sites.