"What in the world makes webmasters think it's okay to fail to maintain their mappings between URLs and live content?"
"I have trouble believing any CMS author could fail to support them"
It almost sounds like you are assuming that The Onion has some sort of duty in maintaining ancient URIs. IMO, they should maintain their redirects as long as deem it necessary - there is a cost in doing so after all. Sure, the cost of not maintaining outdated URIs may or may not outweigh the cost of losing vistors, but that is for them to decide, not the peanut gallery.
I'm sure I'm not the only one that isn't so entitled that I wouldn't forgive The Onion if the links from my 2001 bookmark.htm export fail to land on the original content.
The importance of a URL is how heavily it's used by the web, not how new it is. The cost of maintaining a URL is far less than the cost of maintaining the content behind it. If you aren't throwing your work down the memory hole, breaking every link to it damages the web and makes far more work for everyone else than you are saving yourself.