How did the change in breastplates and helmets contribute to the decline of rome?

The removal of the breastplates and helmets contributed to their military weakening. It weaken them because the removal of it promote laziness and  negligence of training and the such. It also removes the protection against ranged weapons which caused many casualties. Also the armor promoted the stand and fight stance since they can't run in heavy armor,  but the removal remove the physiological method and caused them to run away which caused fronts to fail. All these point to the military failing to protect Rome.