I more or less agree with you. Though, historians in the future may draw all kinds of lines. It may be that Eisenhower is seen as ushering a new system with the, "Military Industrial Speech." In which case, it could be said that the lines were drawn early on and it just took a few decades (or longer) for the executives to figure out how much power was on their laps. After all, the US went from a country without a standing peace time army to having military bases in virtually every country with first strike capacity within Ike's lifetime.
I would say that the rise of the military-industrial complex, together with the founding of the CIA, marks America's 'Gracchi moment' - the moment when it becomes clear that things cannot continue in the same old way as before, due to the objective changes both in America's place in the world and in the relationship between the people and the state. The system can, and will, continue for many more decades under its own inertia, but a fundamental transformation has taken place. Attempting to simply ignore that transformation will just mean that the political system will become increasingly detached from objective reality, will become increasingly
unreal in the Hegelian sense. In my view, this process is already well underway.
Or it could be something else that marks the line.
America's 'Augustus moment', or even its 'Caesar moment' has yet to occur. Hell, it hasn't even had its 'Marius moment' or its 'Sulla moment' yet.
Point being though, I'm not sure that it was clear to the Romans that the system had come to an abrupt change at the time.
Clearly,
something had changed - there was now a single individual who was essentially Dictator for life. Where the Romans disagreed with each other was over the
causes of that change - whether it was caused by the overweening ambition of a few power-hungry individuals, or by fundamental and irreversible changes in Rome's place in the world and in the relationship between its social classes.
I would suspect that there were some saying so, some saying there were changes needed to keep the republic going, and some saying that nothing had fundamentally changed. You already tend to see people saying those things in the US - only when the dust has been settled for a few centuries one could tell.
I agree that some things in history only become clear in retrospect - in fact, historical events only have a 'meaning' when seen in retrospect - and it is only after many decades or centuries have elapsed that it can be seen whether those changes were merely a temporary anomaly, or proved to be irreversible. At the time, many or even most Romans probably believed that the rise of Julius Caesar and then Augustus Caesar was a mere anomaly which could, in principle, be reversed. However, by the time Tiberius became Emperor it was clear that this change was
irreversible.
"Politics is the art of looking for trouble, finding it everywhere, diagnosing it incorrectly and applying the wrong remedies." - Marx (Groucho)