For Ammianus the late empire was not in the process of declining but in that of maturing. In fact "declining into old age, and often owing victory to its name alone, Rome has come to a quieter period of life." He did not, however, pursue that quasi-organic analogy to its logical conclusion, the eventual death of the empire; such a conclusion may have been unthinkable even for Ammianus. Or he may not have reached that conclusion because he, like so many ancient historians, did not ascribe the problems of states to structural changes but only to individual persons who failed.
Ernest Breisach(1923-2016): Historiography: Ancient, Medieval, & Modern. Third Edition(2007, 1. versio 1983), sivu 74.