The other problem is that the connection with Roman trade and Roman arms by working as auxiliaries turned smaller, disorganized tribal groups into bigger, more unified tribal kingdoms. The Romans found these useful as allies against enemies further afield and as auxiliaries, but they were also more potent adversaries for the Romans as well.
My point was that the Imperial system developed as a response to the inability of the Senatorial system to govern the Empire without civil war because any one Senator aiming for supremacy would be countered by the remaining Senatorial class. The creation of a professional standing army created a delicate tripod in the governance of the Empire: the Senatorial class, the Emperors, and the Army. Additional military pressure made that tripod unstable and Rome degenerated into a corrupt military dictatorship.
Let's just hope we don't follow the same progression.
The Senate wasn't an elected body, and the reason for two simultaneous (and term-limited) consuls was to give factions veto power over each other. That was the fake republic, and it was rotten to the core (and wasn't great to the infantry).
Had Rome not been repeatedly attacked by various furriners (the Gauls invaded Italy and burned Rome; the Carthaginians encroached, had a large threatening fulltime navy, eventually invaded Italy) it probably wouldn't have evolved an effective military.
A standing army didn't become a reality until after the second triumvirat broke down (Lepidus was a real weasel by the way) and the resulting civil war ended.
Having a permanent supreme chief executive was a necessity for a former city-state that came to control a large territory. Paying attention to the earlier form -- two consuls, one who was imperator with the real power, the other appointed by the imperator -- worked out quite well. Rome's first conquest was Ostia about 500 BC, and the Empire effectively ended with the Turkish conquest of Constantinope in 1453 -- 39 short years before Columbus sailed off.