The sight safety did not "shield the firing pin and block the hammer." A surface on the bottom of the safety imposed in front of a cutout in the firing pin, blocking the firing pin when the safety was down. The cutout in the firing pin left only about half of the firing pin. If the hammer was cocked with the safety on, pulling the trigger dropped the hammer on the firing pin which was blocked by the safety.* The result was that the firing pins, already weakened by the cutout, broke. That was the main reason for discontinuation of the firing pin safety. (The firing pin was not the inertial type.)
*It wasn't a much better idea some 38 years later when the German Army overruled Walther and insisted on a cheaper safety system on the P.38. What broke that time was the safety, not the firing pin.