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 (: