Hammers are superior to Shields in every possible way. A Hammer is like a verb- pro active - a noun with a punch. What is a Shield? a retroactive "don't hurt me" thing they give like minor female characters in comic strips.
The mighty modern Russian state was borne of the Hammer and Scycle(sp) while the Great English Empire, with their Coats of Arms Shield emblems, melted away....
In ancient Norse there is a saying "Shield bearers thee are of sux"
Thier Thunder God carried a War Hammer- not a War "Shield".
A Hammer is also useful around the house. And even at the office. Whereas a Shield is so much clutter. A practical Graphic that lends itself to War, Industeriousness, Building new things with Workers, Blacksmithing, Railroading ect.
The Shield? Barracks, War thats it.
In the future perhaps the Shield will see some revival in the form of like a space ship shield - however in any environment the Hammer will be found- hammer'n away.