Rome also had decades of civil war where a majority of the ruling class was either a) killed in said wars or b) killed after the wars in proscriptions by the victor. The only people left in the Senate were either weak-willed or allied to the "winner". The proscriptions by Marc Antony basically wiped out whatever resistance was left after the death of Julius Caesar. Couple this with the fact that the populace by-and-large just wanted peace, you can see why Octavian was able to rise.

In relation to the parent point, Octavian also went through great lengths to _appear_ to be acting at the will of the Senate. He was "first citizen" and all rights and protections granted to him here "granted" by the Senate. Whether this was under duress or just abdication for the sake of peace is debated.

There are some parallels between the current situation and the fall of the Republic, but there are for more differences. We are more at the Gracchi stage than the Caesar stage at this point. So there is hope!