The odd thing about the line "Pure as the driven snow" is that its meaning isn't understood the same way it was ages ago.  Originally, "driven" could mean "windblown," but that's not how most modern people would see it. 

When I first heard this idiom, I thought, "Driven snow isn't pure."  For myself as well as most others (when they think about it), "driven snow" conjures up images of packed ice on the side of the road tainted with mud and slush.  We still use it - we just don't think about how meanings have changed.

It's up to you to decide whether or not this is a metaphor for changing times.  Personally, I don't want to dig too deeply into something coincidental.