Ironically, if we look at history "right by birth" is a more moral system than we give it credit for because it gives the singular leader an amount of security in their position so that they less likely to become paranoid and start cutting the heads off of all the competent people around them that could possibly usurp their power.
That was a indeed a common historical argument against representative government, but I don't think we have to take it at face value, because the reality is that even the people making that argument would have been arrested and tortured if they'd advocated for representative government or constraints on royal power, and that's probably a big part of the reason why they weren't.
In fact, a major feature of the ideology of absolutism was the idea that monarchs (and to a lesser extent aristocrats) couldn't be held to any kind of moral standard. Even before absolutism, it's very rare that kings or aristocrats were expected to live by any strict or stringent moral code. In fact, flouting the moral conventions which bind the rest of society was often seen as desirable behavior. Cruelty, domination and excess was part of how a person with power was expected to behave.
When Rome expanded larger and larger, it outgrew what it's system could handle with their ability to communicate.
This is a good example because we kind of have to qualify what we mean by Roman expansion. Rome was a city-state. It never really stopped being a city state. Eventually what was now Italy came to be more-or-less under the direct rule of the Rome itself, but the vast majority of what we call the Roman Empire was administered by semi-independent provincial governments and Roman client states.
And actually despite the modern mythology of the Empire bringing order to the chaos of the Republic, the reality is that the political system was pretty much continuous and that things got worse, not better, during the Imperial period. The Roman Empire was profoundly unstable, so much so that it ended up being split into four parts ruled by separate Emperors, which saved it in the short term but also created the political division between east and west that would ultimately break the whole thing apart. Even so, it's worth noting that the "decline" of the Roman Empire lasted longer than the USA has existed.
In fact, the Roman Empire is kind of an example of why a consent-based monarchy doesn't really work, because without a clear line of succession, if any popular or successful person can theoretically claim the throne and if the mechanisms that ensure an ordered transfer of power are too weak, the entire thing collapses into massive civil wars every couple of generations.
