The gladius wasn't developed as a dueling weapon. It was used as part of a system, which included disciplined units fighting behind 3ft shields. And it worked for centuries against Gallic and Germanic tribes swinging much longer swords.
The Japanese were absolutely unafraid afraid to die in WW 2, so we accommodated them. Romans fought and destroyed lots of people who were unafraid to die.
True, but they were fighting on islands, so maneuver was not an option. More or less like a cage fight, with them stuck in the cage and the United States Marine Corps running around their cage.
The Romans didn't have much luck with the Parthians, did they?