Barbarization?

[Nathan Ross]

For the Danube, it is quite clear that until the defeat of Adrianoplis, the main defense line was organized around roman troops.

I wouldn't say it's that clear... Ammianus Marcellinus, writing about Valentinian's last visit to Pannonia in AD375, says that Carnuntum was 'deserted and in ruins' - and this was supposedly the main base of a legion! We don't know if he meant then or at the time he was writing, but the Danube defences appear to have been long neglected in favour of a second line of fortified cities; when the emperor needed a secure winter camp he was obliged to fall back on one of these. Then we have the massive new fortification at Keszthely Fenékpusztát on Lake Balaton - big enough to house 4000-6000 men, but not even mentioned in the Notitia Dignitatum. This suggests that the ND's Pannonia list may have been obsolete before the end of the 4th century, and the situation on the Danube frontier already quite transformed.

The troops were removed. Sent to Italy? Italy would have been full of men, and it wasn't.

Helpfully, we have the figure of 30 numeri for the force that Stilicho managed to gather at Pavia. That would be about 18-30,000 men by most estimates, and appears to have included the entire field army of Italy plus whatever troops he had drawn from Gaul, Britain and elsewhere. Clearly the army was a shadow of the force listed in the ND, although whether this was a temporary situation or a more lasting one we cannot say.