Good topic - have long considered it - will enjoy reading others thoughts. A quick start:
Evergreen = truth.
To use Syzygy's "battles" example: unless a significant error in fact or perception is found the book remains valid.
Similarly, a treatise about Mars written a hundred years ago, describing canals, crops, and probable cities, is now only a curiosity. It lost it's "evergreen-ness" once new truths replaced the old.
That is not to say that anyone is interested now, or will be more or less interested in the future. Just because something is true doesn't make it popular. Just evergreen.
The exceptions to this all devolve to conspiracy theory. Conspiracies real and imagined are forever evergreen. Especially in America.
Just because something is true and evergreen does not make it popular or even relevant (except to historians and archivists).
Something is either true or not. Very binary.
The difficulty is determining popular evergreen topics. Find a blog that has been around for a couple of years. Go back in the archives. Some entries are still valid/evergreen but past their sell-by date. Some entries have been proven wrong by later developments. Others are still as pertinent now as then.
The more general the topic usually the longer it holds: "The Marketing Guide" will likely outlast "How To Market Using Alta-Vista". The past will outlast the present simply because the past is already played out: "The Napoleonic Wars" is likely better long-term than "The Second Iraq War" as facts/perceptions change as information over time plays out - although for immediate traffic and popular interest the converse is true.
This brings up the fluid nature of the web. If I have a page that later facts prove contains inaccuracies I can update by crossing out the old and incorporating the new. Or by pretending I was right all along. The topic can be evergreen and yet change.
Evergreen does not require stagnation.