This is my opinion of course, which I offer as a counterpoint to your own. This is the period which I find most fascinating.
Quoting one author as "respected opinion" may be true, but that's still just an opinion of one person.
The FoG list writers aren't the only ones who think that metal armour became much less common amongst Roman foot well before this period. In that respect, FoG is perhaps relatively lenient in allowing everything to be armoured up to 425AD, long after the crisis has really hit (which I would put at 408 rather than 378). I don't think there is by any means a clear consensus either way, but that certainly indicates that the evidence is not so one sided as you believe (or claim).
>Regarding Roman infantry for example the list implies that Roman infantry at that time were no more armoured than western barbarian foot (ie minimal)
I'll quibble with this point. Roman foot may well have been better protected than "minimal", but without it being enough to qualify as "Armoured". For what it is worth, my Auxilia are unarmoured (unless you choose to assume they have something slimline under their tunic), my legionaries have a bulky cuirass but non-metallic. That of course is an interpretation dating back to the 1980s and "Armies and Enemies of Imperial Rome", and the figure range I bought at the time and still buy today to expand the army in consistent fashion.
From what I have seen, e.g. when I used to spend a lot of time reading
www.romanarmytalk.com, a lot of the arguments in favour of continued use of armour were of the "well they had it before, they know how to make it, why would they stop using it?" variety, which kind of misses the point. and the fact that a lot of those who particupate there are hard-core re-enactors made me wonder if there wasn't an element of wishful thinking intruding - I'm sure they would much rather be wearing metallic mail (and in the real hard-core cases, experimenting with how to make it using realistic historical materials and processes) than something lesser.
Artistic representations apparently not showing metallic armour are one thing (you can choose to explain these away as artistic convention if you like).
Contemporary accounts are another. Unfortunately, they are a bit thin on the ground. And it is unclear when Vegetius wrote - the earliest possible being 383, so I'm not sure that a "temporary" shortage after Adrianople is particularly plausible. At 383 most of the empire is still in pristine state, a relatively small proportion of the army has been destroyed, and in any case 5 years should be time enough to make some new equipment.
And if Adrionople should cause significant difficulties in supplying the troops with armour, how much more so the many serious defeats of later years?
I'm not sure what archeology tells us, but I have the impression that relatively little armour turns up at later-dated sites.
And then there's the circumstantial/speculative evidence...
By 425 large chunks of the empire have been lost or temporarily regained after being fought over and plundered for years. Large numbers of Roman units attested in the ND seem to have mysteriously vanished from sight - e.g. where were the Roman forces in Gaul and Spain as the Vandals and pals seem to have effortlessly marauded over a large area at will for several years? Destroyed in numerous small scale battles that were not recorded? Already gathered into Stilicho's forces in Italy by this time as an emergency measure? Pulled forward to the Rhine frontier and destroyed at the inital crossing? Or remaining scattered in their barracks and unable to put up co-ordinated resistance simply because of the political disarray at the time?
Move over to Stilicho. He manages to defeat Radagaisus, but then seems only able to achieve a stand-off against Alaric with the help of large numbers of Foederati, impressed into the army having been captured in the earlier battle. And when they desert to Alaric after Stilicho''s execution, the Romans can do very little to defend Italy other than hiding in Ravenna. Even the Eastern Empire at this time seems unable to provide much assistance, a few thousand reinforcements arrive at Ravenna by sea and that's about it. "Proper" Roman units seem to be hard to find, reasons disputed but losses of men and equipement in battle must be a major part of it, probably combined with financial difficulties making it hard to replace, train, and re-equip troops at the necessary rate.
Obviously the Eastern Empire did recover, but perhaps not fully until around AD500.
Let's just say that the Roman army had a very patchy record in engagements fought after 408, victories tended to have short-lived effects, and were often achieved in any case with the assistance of large numbers of "barbarian" *allies*. And by 425 the empire is much reduced, and it's only another 5 years before he beginning of the loss of western North Africa, a very serious economic blow. There's not much in the historical record that would cause me leap to the conclusion that at this time Roman troops were still significantly superior to those of their opponents, either in training or equipment. And quantityof troops had greatly reduced too. The FoG list does, as you say, reflect this by reducing numbers available and forcing quality to be reduced on average. I also think it is a reasonable interpretation for the list to insist that not enough of the infantry would have metallic armour, and those that did would not be concentrated enough, to qualify as armoured in game terms.
This is why the Foederati list is my favourite - you have to adopt different tactics if you want to be successful, and rely on different troops. Winning at FoG with very few armoured troops is an interesting challenge. Sometimes I even manage to do it!
So the flaw here in FoG, again in my opinion, is not that the list is inaccurate; it's that armour is over-powered/under-priced. But that's a wider problem than just this list!