I agree with that. We are on somewhat different tangents here. Yes, there are many worthy individuals and events that have not yet been filmed. When I said that all the great themes have been revisited, I meant by "themes" the great moral arcs involving friends, family, fractured relationships, faith (or lack thereof), commitment, betrayal, courage, honor, sin and redemption, reconciliation, forgiveness, self-sacrifice, etc. Our behavior is highly patterned. We see the same stories repeated over and over among people of different times and places. One of the reasons we endlessly retell such stories, and hunt for new examples, is because they speak to universal truths.
I've been reading a lot of reviews in recent years. One of the recurring criticisms of conventional dramas is, "yes, it's a good story, but it's nothing we haven't seen before." That doesn't mean that the particular individual or event has been filmed before; it usually means that the character arc is familiar territory, as it should be if we are dealing with reasonably normal people under stress. Filmmakers who try to find a new twist are sometimes tempted into bizarrely convoluted plots or to ever more grotesque extremes of (often dysfunctional) behavior in the search for novelty for its own sake.
P.S. To use your example, the usurpation of Phocas is thematically an oft-told tale. As far as I know, Maurice and Phocas have never been the subject of an American film. But an embattled and desperately stretched empire? A competent, soldierly emperor betrayed by a trusted subordinate? Ambition, treason and murder? The human tragedy of the “game of thrones?” The catastrophic consequences of the treachery? These themes have been revisited many times, as loyal subordinates turned backstabbing traitors are legion.
Heck, isn’t this the theme of MacBeth? And 101 other films? It is a universal theme, and it can be revisited endlessly through the lens of many similar events, both historical and fictional.