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Indeed. Especially since Friedman's quote from Lewis Mumford, "[e]veryone aimed at security: no one accepted responsibility..." is a libel against the late Romans, many of the best of whom eschewed personal security and willingly accepted mighty and crushing responsibilities for trying to preserve the empire. You would be hard-put indeed to find any evidence at all that the generations of Stilicho, Aetius, Theodosius, Justinian, and Belisarius were any less public-spirited or brave or far-sighted or responsibility-accepting than the generations of Cicero, Caesar, Augustus, and Tiberius.

I would say that ultimately two things brought down the empire. The first was the repeated heresy-hunt by emperors and patriarchs against fellow Christians--the heresy-hunts against Arians and Monophysites and others so that, when enemies like the successors of Mohammed showed up, nobody in Egypt or Syria wanted to fight to remain in the empire so that they could get persecuted again. The religious tolerance practiced by expanding Islam was a major string to their bow.

The second is related to Goldsworthy's musings on the opening-up of the contest for power, but not quite the same. Goldsworthy says that up until 200 or so you had to be a senator to be a field army commander, and so only senators could make a grab for the empire by gaining the loyalty of their field army. Goldsworthy further says that after 200 emperors thought that if they kept senators from army command then they wouldn't have to worry about frontier generals making a bid for power--for who in Rome would agree to be ruled by some upstart whose ancestors had never been a senator? And Goldsworthy says that was a big mistake: it multiplied potential contenders for power and the damage done by civil wars rather than reducing them.

I think it is more complicated than that: after all the empire, even in the west, held on for more than 200 years after the purging of the senatorial class from army command.

What appears to have killed it in the end was the rise of a set of military politicians who were both Roman generals--hence able to get segments of the Roman army to follow them and know how to use the Roman logistical infrastructure to support their troops--and barbarian war chiefs whose warriors would follow them for "ethnic" and "ethnogenesis" reasons as well. Such leaders turned out to have a big advantage in the fifth century as they combined two sources of power. And in the end some of them decided that they would rather try to be secure as barbarian king of a region carved out of the empire rather than aiming for imperial dominance. Flavius Stilicho, the Vandal. Flavius Aetius, not a Hun but somebody who had been raised among the Huns and had carte blanche to raise Hunnish armies--when he was not fighting Attila, that is. Alaric, King of the Visigoths and also Magister Militum per Illyricum. Theodoric the Amal, King of the Ostrogoths and also Magister Militium per Italiam. That was a change made possible by the (centuries before) purging of the Roman senatorial class from army command. But it was not the same thing.