The fall of the empire was due to a number of factors, many of them not military. Goldsworthy argues that Roman legions of the fourth century AD still got the better of their opponents most of the time. The problem was not the effectiveness of the legions, but the fact that there were less and less of them, and they were depleted by endless civil wars.
Methinks you are idealizing the Gladius Hispaniata too much. It was a good weapon, but hardly the only or even major factor in Rome's rise. Unless you prove otherwise, you cannot blame Rome's fall on its replacement.
Bookmarks