Yes, true. They also later started adopting traits from their conquered provinces as well such as germanic/celtic pants (much to the roman populaces uproar), and persian inspired limb armour for the legs and the arms. But the 4th to 5th century armour still seems cheaper and less protective, with the exception of the legs, to me atleast.
The ridge helmet (Intercisa on this picture) is a perfection of the galea. It may look simpler, but it is superior in both manufacture and protective capability and tactical awareness. The older helmets had just too much bits hanging out which could be used against the soldier; they were good helmets, but needed some work. Mail armour is cheaper, yes, but it is still very effective when combined with the subarmalis (which is a Roman gambeson). Just because it's cheaper doesn't mean it's crap. Mail is far easier to fix than plate armour and has loads of good qualities (:
I must also reinforce what Fall3NAiRBoRnE already said:They never reverted to chain mail ! This is one of those basic misconceptions repeated again and again on the internet.They used it all the time before during Empire and it seems very likely that it was the most common type of armour in the Imperial Roman army not that plate armour so much overhyped in pop culture.And more than that-we actually have some evidence suggesting soldiers preffered older types of armour over plate armour.Other error is speculation that chainmale was lighter-in fact it is more heavy than plate armour.Up to this moment nobody also proved that chain was somewhat considerably cheaper than plate.Actually it may be that chain male was more expensive than plate(arguments for it exists).It should also be said that Roman army didnt only started with adopting barbarian equipment during late antiquity-Roman army did it all the time before.Almost everything from Roman military equipment was of barbarian origin and we also know from the Roman writters themselves that they considered their ability to adopt useful things from their enemies one of the key factors for creating their huge empire and were proud of this adaptability.