Created does not equal last read. One could have created things then never looked at them. Modified would imply last read so obviously that content was useful at that time I don't believe that I have stated that zero human intervention is necessary to cull information that is no longer needed. But it doesn't have to be overly complicated either.
For example, generate a list of content that has not been read in a user-determined range of time. Then it would be up to the user to decide if it was still useful or not. Seems simple. But not if Evernote's not keeping track of the last read date/time.
Just because you cannot think of how to implement something doesn't equate to "it can't be done". Actually, it has been done and been done for years. Not at Evernote though. And if one was interested in monetizing storage, then there's really no incentive to do so. Thus the point of my initial post.