I feel this is a common trend for any social media. I feel similarly as you regarding how Reddit has changed, and while some of that was due to changes to the website, I'd attribute most of it to an increase in overall popularity. Reddit used to be a collection of fairly niche forums, but has really ballooned into almost Facebook levels of social media. It seems like that's just how the internet works, and eventually new, more niche places for people to discuss will pop up and the cycle will continue.

There probably aren't fewer smart/interesting people on HN than there used to be, but its possible they're just less likely to be seen often or voted to the top.

Of course I say all this as someone who started spending time here relatively recently, so its pure speculation on my part.