The Romans did subscribe to a sort of great man political theory, at least during the phases of imperial expansion. You could see it in the way the body politic would conceive of fortune and misfortune: they'd blame the individual moral character of a given emperor if things didn't go well, and a major military defeat, or a string of defeats, could spell certain doom for a reigning emperor or military commander. By the same token, a major victory or a series of victories could make the political fortune of whoever managed to get in front of that parade.
It is true that there are individuals with rare perceptions and talents, and in a world where conventional warfare played a huge part in shaping the development of empire, an exceptionally talented general could alter the fortunes of an entire empire, and would be encouraged and rewarded as such.
Of course we know that for a gifted and talented individual to actualize their contributions, the resources and opportunities must be available; the pathways must be open to their flourishing. The great historical figure is as much a function of their own localized vitality as their embedded position in the flow of history, and the arc of their own lifetime is expressed in the directionality of the historical era. Further, the celebration of an individual requires a collective valuation: who has the power to deem what is important?
There are many good things in life, but as we are all frustratingly aware of at this point, a given society chooses a more limited expression of the good, which is all the more true in proportion to concentrated power and the concentrated utilization of that power to determine what the good is and impose that determination on the ruled.
Nevertheless, a unified collective ideology - whoever crudely it cleaves off the deeper complexities of reality in favor of a functioning narrative - actually works in the sense that it is communicating with its environment to concentrate power for its wielders, coordinating the efforts of all of its adherents - whether by force or no - in order to achieve a given result.
The belief in the great man gives rise to a vast institutional apparatus: the processes of the transfer of power and its wielding, the development of the individual character of the emperor and the deference to that character, the geopolitics of societies of similarity and difference, and so on. If everyone believes the emperor is right, then everyone naturally coordinates their efforts and organizes their actions in accordance with a guiding protocol that is manufactured out of the activity of the emperor and all of the family and handlers and advisors and allies and the like, and the society is able to move as a coherent whole and interact with other societies as a coherent whole. This is always imperfectly realized in varying amounts, but nevertheless the ideology works.
The better the ideology works, the more suited to its geopolitical, domestic, ecological, and thermodynamic environments, the more the ideology is reinforced and entrenched, making it more difficult to dislodge when conditions change.
This principle also works in times of contraction, but the rationale is reversed. This was one of the major ideological changes that took place from the decline and fall of the Western Roman empire to the major upheavals in the seventh century that the Byzantine Empire experienced. The process was slow and took centuries, but it was nevertheless a distinguishable process with distinguishable gradients that we'll briefly look at.
An inevitable biproduct of a contracting empire is the ubiquity of dysfunction and failure. Civil wars do break out after the inevitable string of economic and military failures, and one leader is replaced after the other, but eventually the body politic figures out that its problems are much more universal and intractable, and that blaming an individual emperor no longer makes sense if the society is to continue to function without being embroiled in perpetual civil war.
You can see the great man theory transition into the fallen man one: the increasingly desperate emperors begin to identify themselves with the developing idea of the Christian god, as god emperors with divine right to rule. But even this universalization of the individual begins to buckle under the pressure of ubiquitous collapse.
Here the religious leaders take over, and describing a whole people as immoral and corrupt and as a whole collectively responsible for the misfortunes of the society, were tasked with devising the correct moral and spiritual doctrines which were to be implemented by capable leaders. Here you can make out the preservation of the patriarchy and the great man theory: fallen man has advanced inevitably into catastrophe because of his very character, but it was up to the religious patriarchs to describe the correct pathways to moral and spiritual salvation, and these pathways were to be embodied and expressed in particularly rigorous and faithful individuals and then emulated by the masses. The very idea of the Christian god itself was of a monarchical figure that had to be obeyed.
As the fallen society stabilized, the organization around the individual was preserved by shifting attention away from the individual, but which would return as exceptional individuals led by example and the attention would again turn to the individuals doing the leading, with the great man theory to re-emerge in the process.
