This is a good question that probably deserves more thought and effort than I can apply to it. I would say that when HN was at its peak, the overall vibe of the commentary reflected the perspective of people who people who built things, while the overall vibe today reflects the perspective of people who like to go online and bitch about things.
There’s a famous email exchange—I’m sure you’re familiar with it—where someone writes to Steve Jobs complaining about a bunch of things Apple was doing, and they go back and forth a few times, and Steve Jobs finally gets annoyed and writes back, "By the way, what have you done that's so great? Do you create anything, or just criticize others’ work and belittle their motivations?" HN these days is absolutely full of people who don’t seem to do anything but criticize others’ work and belittle their motivations.
I’m not saying that HN never had unfair criticisms in the good old days—the “middlebrow dismissal” has been a trope here for a very long time—but we didn’t use to have entire threads filled with nothing but middlebrow dismissals. And we also had tons of people criticizing Apple, but then again most of them were complaining that Apple was actively interfering with their attempts to create things (e.g. the uproar over arbitrary and unfair App Store moderation policies).
The clearest indicators of decline to me have been the signs of evaporative cooling. Maybe I’m falling into a different common fallacy by saying this, but I do think HN was a lot better when the old regulars—yourself included—were more active. I don’t exactly blame you guys, but it is an indicator.