High Imperial Roman army vs Late Roman army

[Flavivs Aetivs]

That's not true. The Western Army was hit hard but we know it continued to Function, the Italic Army alone numbered 28,500 men, the Gallic Army 34,000 men and this was in 395. It wasn't until the West began loosing Territory in 406 that the Armies began to decrease in size, as the income lessened. The empire had no shortage of soldiers, they just had a shortage of Romans to serve as soldiers. Good Generals like Aetius and Stilicho respected barbarians as recruits, especially when they were filling up the professional "Roman" units instead of serving under their own leaders.

People overestimate the damage taken at the River Frigidius. The west took quite a beating, but not a crippling one. How else were Stilicho, Constantius III, and Aetius able to hold the empire together? All of them lost their foederati as those groups rebelled against the Romans, but all of those foederatic groups were defeated by Roman generals leading Roman troops.