> Users don't normally construct urls by hand

You never typed google.com into the browser? I doubt it. Maybe you just mean "construct" as in edit the url to access another site - well, that's still a perfectly normal use-case. I regularly change reddit urls to old.reddit because it gives me a better user interface. Or access a subreddit by adding an "r/subname". Sure, those aren't alphanumeric IDs, but that distinction is meaningless. Some unique IDs on the web do actually consist exclusively of english words. And some numeric IDs are harmless page numbers or pagination info.