In the spirit of the TheMapHasReplacedTheTerritory, Let me offer: TheTerritoryIsntWhatItUsedToBe.

Meaning -- At a given time, a representation, map, chart, description, definition, etc. may be a good illustration of of an entity or process, but given the passing of time and the change that it introduces, What was once the territory, is no longer what it was.  (ala PersonalComputer, Laptop, Linux, Unix, Windows, WikiWiki, the old neighborhood, that brand new car without a scratch or dent, your opinions and beliefs, The Internet, your waistline, the picture on your driver's license) It just isn't the same! --DonaldNoyes

''Okay. Is there some grandiose wisdom to this that I'm missing?'' 

PerhapsOrMaybeNot. Try HorselessCarriageThinking to pique the imagination or sense of humor.   

''Is this by chance a reference to the salesmen scene in '''The Music Man'''?'' Or maybe Tin Men.

Observation and comment is not meant to be "grandiose wisdom". If you are looking for this in every page you read, and find it, let me know where such a site exists, since I would like to visit it. This is merely a metaphor, which may be applied to many situations and scenarios with equal validity. Even to such things as the Music Man or the Wizard of Oz. 

Try thinking metaphorically from time to time, it is a welcome relief from SituationNormal. 

"First, a bit of etymology. The word, 'metaphor' is a Greek composition, consisting of 'meta' (over or beyond) + 'phor' (to carry). So, literally, to carry beyond. How does that map to our current usage of the term? I'm not sure, but I think it means we take the literal, directly observable, and treat it as a symbol for something else, thus carrying it beyond its literal meaning."

''In the sense of this page, if it is carried, it is carried by the flow of time.''  Expressed by such statements as ''you can never go back''. What was before is either moved, destroyed, converted, or disappeared.

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Does "TheTerritoryIsntWhatItUsedToBe" mean that even should you make a perfect map,
you'll find that the thing you mapped changed,
so your map isn't perfect any more.

This could be an XP view of conventional requirements gathering:
You can't get it right.
And even if you could "get it right"...  by the time you do so, TheTerritoryIsntWhatItUsedToBe.
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Or to put it in a SimpleMinded phrase ''Time changes everything'' (from what is "was", to what it now "is")
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CategoryMetaphor