Theodosius and Valentinian - A Brief History

At no point does it appear that Arbogost intended to overthrow the young emperor, because what would have been the point of that? The general had all the power and prestige he could ever hope for without worrying that a strong imperial hand would check that power. Valentinian was left to languish in his impotent isolation. Ruling an empire was serious business. Nobody wanted the inexperienced Valentinian screwing things up.