When did the Roman Army decline?

[Flavivs Aetivs]

Well, you can't say we don't have any other eye withness accounts other than Ammianus. Merobaudes and Claudian, the pangeyricists of Aetius and Stilicho respectively, were eyewitnesses to their campaigns (Merobaudes was even a General). That's just one example. Sidonius Apollinaris describes roman soldiers under Avitus and Majoran as being clad in Mail and Scale Armor, and that was when the Empire was on its last leg.