Some historian once wrote that the basis behind both the Roman social structure and in effect Roman politics was warfare.

The idea is basically this: like all states in early stage of development the only way to gain wealth was simply by taking it away from others, the Romans themselves didn't have much else than their farms and to 'make a decent profit' they used to take it from their neighbours. So far nothing unusual, this practice amounted to the institution of a form of client tribes bordering the Romans paying some tax but not much since those tribes didn't have - like the Romans - too didn't have much else except their farms.

But what this did gain the Romans was this: more soldiers, larger armies. And as the only significant way of enriching the state and thereby gaining social status was to be victorious in war, succes on the battlefield became a key too succes in politics.

However, due to the fact that this practice also created a lot of unemployed (the proles) - because the powerfull senators started to buy all of the land in Italy from the local farmes, financed by the loot they had received when commanding armies, and those farmers couldn't get a job because, as, in the same time, slaves started to be used instead - political reforms were needed. (Remember those civil wars Livius told about in Ab urbe condita?) Those reforms gave the people much more influence in political disicions, through offices like Tribunus Plebis and the fact the from now on one consul had to be memeber of the common people (or plebs in Latin).

This resulted in the practice where Senators started to buy votes from the people, to get elected for an office. But since that portion of the people that didn't have a job, else than being a soldier in times of war, kept on growing, it became more and more expensive to get elected. Once elected all your worries were over, you 'simply' started another war against some small/medium sized tribe and cashed. But as soon as you lost, this nearly always meant a financial bankrupt.

That amounted to a state where politics was an nearly impossible enterprise, if you wanted to achieve something. Only those who were extremly rich and had very rich or very popular (among the electorate, of course) friends, could then effectively get into a certain position which enabled them to wield nearly absolute power as long as their alliance remained intact. This basically was how the First and Second Triumvirate worked.

And that meant that as soon as such an alliance was in pieces, Rome was in the midst of political, social, and military chaos. Realising the latter, it is no surprise that one man, who had gained himself absolute sway decided to put an end to this, and turned Rome into an centralised Empire. Which it sooner or later was meant to be. Constant warfare, without having a strong economy that creates tons of surplusses even if it is isolated from all possible trading partners, (which untill the Industrial Revolution didn't exist anywhere in the world) is only possible with a minimal chance of political or social upheavel. And therefore, only a state where internal strife for status is minimalised can achieve this.

The Roman Republic couldn't provide the needed stability, and therefore wasn't going to last anyway: even if Caesar and his friends didn't gain the later Empire for themselves, someone else would.