Am I to understand somebody, somewhere, believed "knapsack" to be an obsolete term? That would seem a symptom of not being widely read, or something.
A knapsack itself is no longer the cutting-edge state of the backpack art, true, but all it is is an unstructured sackish thing on shoulder straps. We still have unstructured sackish things.
Looks like I have deeper linguistic roots than some I could name.