I could see paper money, such as Confederate dollars being worthless, but gold? Surely not. Of course it could have been melted down and reused if not for money but certainly jewelry, goblets, and such.
With the departure of Roman garrisons, and the arrival of barbarians (whether from Scotland or the mainland), the people with some wealth in gold & silver most likely hid it in the ground so as to make their escape easier and reduce the chance of being robbed and killed for it. They probably planned to dig it all up after the crisis passed and Roman law returned...trouble is, it didn't happen....