This is a movie simplification. The Romans copied battle styles from local armies during their conflicts. When Romans started having money problems they started outsourcing their armies to local people and they stopped their public works. When the army started getting less relevant to local affairs and only a tax imposing machine, the locals would turn against the Romans and join the militia. The Germanic leaders were also trained in the Roman army, and the "barbarians" pretty much knew how to conduct warfare and they also knew their weaknesses. So they fought a war of attrition, that forced the Romans to spread out. Soon enough the Romans knew they could not sustain their wars. So the "barbarians" tried different things until they found fight patterns that Rome did not have the technology to beat in a economic way. The Roman nobility had not lost it's privileges but nobody was interested in fighting, and they soldiers were only doing it for money which was not even enough to retire afterwards.
So it is totally irrelevant with the current situation.
So it is totally irrelevant with the current situation.