Are you saying that brilliant emperor of the 2nd after Christ century could have restored the Roman republic or that brilliant consul of the 2nd century BC could have become king? I doubt it.
To every thing there is a season, and a time to every purpose under the heaven:
A time to be born, and a time to die; a time to plant, and a time to pluck up that which is planted;
A time to kill, and a time to heal; a time to break down, and a time to build up;
A time to weep, and a time to laugh; a time to mourn, and a time to dance;
A time to cast away stones, and a time to gather stones together; a time to embrace, and a time to refrain from embracing;
A time to get, and a time to lose; a time to keep, and a time to cast away;
A time to rend, and a time to sew; a time to keep silence, and a time to speak;
A time to love, and a time to hate; a time of war, and a time of peace.
(Ecclesiastes 3)
Ecclesiastes had his opinion, to be sure.
Could a brilliant emperor of the 100s restored the Republic?
Probably he could have restored a republic, but it would have had to have greater suffrage.
Could a consul of the 100s BC have restored the monarchy? Probably not.
I don't think these things are like a reversible chemical reaction. People definitely prefer republicanism over monarchy, and democracy over republicanism. Once people get a taste for power, it is very difficult to take away power and privilege from a large group of people. But the converse is not true. A particular powerful ruler willing to sacrifice power CAN give it up - nothing stops him but himeslf.
Individual men only very rarely do that, but some do. George Washington comes to mind (Buonaparte's sneering astonishment "They want me to be Washington" is more typical). On the other hand, bodies of men don't just give up their liberties.