I wonder to what degree Roman military vitality, or the desirability of military service, simply boils down to the republic/early empire's "offensive" posture, vs. the later empire's "defensive" posture.
Up until the empire stopped expanding, fighting successful wars of expansion led to great booty and wealth that the soldiers shared in to some degree. It was about profit. The later defensive wars were just about fending off invasions. I know there were some spoils in successful defensive wars, but not in nearly the same degree. I imagine that conquering a Hellenistic kingdom in the East during the late Republic brought in way more spoils than repelling the Allemani in the fourth century.
Perhaps the citizenry were softened by years of prosperity and other social factors, but perhaps military service also became a "why bother?" proposition, if all you got for it was your regular salary, and an unpleasant, dangerous lifestyle. Was the risk simply not worth what it was in previous generations?