I'm talking out of my ass here, but I can imagine that the focus for this software system was very sharp and hasn't changed (much) in the past 20 years. When you have a product with tight focus, you can polish the shit out of it and make it last 20+ years. A bit of the Unix mentality.
Most products today - and stuff on HN is at the forefront of that - is much more about selling a product to a lot of people, often in highly contested markets. That is, if you make a product, focus on just the core and don't do anything else, you'll stagnate and die.
Then again / on the other hand, there's Dropbox whose core functionality has not changed as far as I can tell for a decade - it still does the exact same thing as when I first installed it. Spotify whose IPO was today that doesn't seem to change its core model. In both cases though, I don't know where they put all their development capacity; probably on back-end systems / scalability, marketing, and new applications (like dropbox creating a google docs competitor).
Most products today - and stuff on HN is at the forefront of that - is much more about selling a product to a lot of people, often in highly contested markets. That is, if you make a product, focus on just the core and don't do anything else, you'll stagnate and die.
Then again / on the other hand, there's Dropbox whose core functionality has not changed as far as I can tell for a decade - it still does the exact same thing as when I first installed it. Spotify whose IPO was today that doesn't seem to change its core model. In both cases though, I don't know where they put all their development capacity; probably on back-end systems / scalability, marketing, and new applications (like dropbox creating a google docs competitor).