Making the HN URLs more human readable would also make them much longer. However, the longer URLs could provide a form of site structure if the URLs are 'editable' e.g.
https://news.ycombinator.com/2016/05/25/regressive-web-apps
Then, if you edit the URL, you can get all stories for today:
https://news.ycombinator.com/2016/05/25/
All stories for the month (which would require pagination):
https://news.ycombinator.com/2016/05/
And all stories for 2016:
https://news.ycombinator.com/2016/
And finally, back to the home page
https://news.ycombinator.com/
Note: all these pages don't have to be explictly linked or displayed on Hacker News, they might appear only when someone edits the URL in their browser.
Edit: Just to add...here's a lengthy, but very detailed post by an ex-BBC employee about using URLs to model the complex data for BBC programmes
Designing a URL structure for BBC programmes
http://smethur.st/posts/176135860