The real cause for the downfall of Rome was the debasement of currency because it led to price controls which suffocated the economy.
Ironically the more corrupt eastern part of the empire evaded many regulations and thus saved its economy from total destruction.
However, even if the Roman armies became smaller and smaller their military skill was still formidable. Thus a brilliant intellect like Julian could still beat the barbarians.
Julian could have saved the empire but he too believed in price controls. Furthermore, he did not understand the power of Christaianity. Thus in the end Julian doomed the empire.