I assume the words classic and classical have a basis in the word class — which is to say, of a category. Why do we use those words to mean old or historically important?
Here's a shot at it, not sure if this is accurate but is logical.
Two definitions of the word can combine to form this meaning of Historic.
The first is
The second is
I would say, the classic model is also the highest quality model. You can't know the classic model unless it is in the past. Thus, the usage of the word describes something of high quality, in the past.
A classic model for economic growth would be one that is proven to work because of its high quality, It is possible that the fact that it has taken place in the past serves its base for being an historical word.
These and other definitions of classic: dictionary.reference.com/browse/classic?s=t
Historically important usually has some values to the place or item in place. Values as in something to do with its history and it being old.
Definition of classical
Definition of classic