Bertil, I have cited the fact that the Romans dedicated numerous coin issues to the Sarmatians as indication that they may have regarded them as fairly important until the mid-5th century. These issues span a period of some 250 years, while only 4 small issues were made to account for the Goths and none was made to account for the Heruls. The Romans had the habit of accounting for virtually every military engagement that could vaguely be interpreted as a success with the minting of coins. Thus, we have coins for larger people like the Skythian Goths, Alamanns, Franks, Parthian, Iudeans, Gauls, Britains, Sarmatians etc. However, the Romans never bothered to issue coins alluding to Heruls, which can only mean that they regarded them as too insignificant.
Coins are after all written documents.
Note, their are several Roman Emperors and historic events of which we only know through coin inscriptions. Maybe you want to expand on the reason why you think that they are not very significant. The question of the reason for the downfall of the Roman Empire is a very very big one. While people like the Goths certainly played a part, the reasons for the fall of the Roman empire are far more complicated and derive from socio economic and internal political factors. You seem to have partly misunderstood my contribution.