the intrinsic difference between a mallet and a hammer is it purpose, hammers are hard designed to hit as hard as possible with out damaging the hammer, mallets are soft( er) designed to deform so not as to damage the thing being hit
So basically what you're saying is that it's the intended use that dictates the design, and influences which of the available materials is most suitable?
And that later, the properties of newly available materials can allow changes in the design