What would be the impact if the Optimates won the
Roman civil war
You would have to explain how eleminating Caesar will end the phaenomenon of powerful governors/generals use their popularity with the legions to march on Rome and seize power. Sulla did it, Caesar did it (and loses) - but what will stop the next generation of ambitious men from doing the same?
The next major wars of the Republic would be the war against the Parthians in the 30s, and the war against the Cantabrians in the 20s. Furthermore, the Senate will have to set up a defensive force in newly-conquered Gaul. These operations will involve large concentrations of troops under a proconsul, and the opportunity of great victories. Victories mean booty, loyal troops and popularity with the plebs. The Roman Republic had problems to cope with Scipio Africanus; the Republic couldn't stand Caesar; now imagine how a proconsul having conquered Mesopotamia from the Parthians, returning to Rome in triumph with the loot of Babylonia, would influence domestic politics!
The Roman Republic was conceived to govern a city state of some thousands citizens. As long as every man was at the same time peasant, soldier and citizen, the system worked very-well. As a peasant, he could earn his living and didn't challenge the leading role of the Roman aristocracy. As a soldier, he was disciplined and experienced enough to protect his city against foreign aggression; but when he was on military duty, he never forgot that he was a citizen, that all his officers were elected by the people's assembly and that they had to obey the laws. Social tensions were reduced by the loot distrubuted among the warriors and the founding of
coloniae.
This systems effectively collapsed in the 1st century BCE. Domestic politics were now influenced by a large number of impoverished citizens who had more interest in economic relief than political reforms; the discontent masses could be used as a weapon against the senatorial aristocracy. At the same time, political participation decreased significantly. In 70 BC, there were almost one million Roman citizens. Only a fraction of them, maybe up to 100,000, participated in the assemblies. I assume that there was almost no popular attachment to the republican government. Why should they cherish and defend political rights they couldn't/didn't use? If anything, the political life took place in the many
civitates,
municipia and
coloniae. Local politics attracted a lot of interest (see Pompeii). But I doubt that many Roman living outside of Rome were very eager to defend some abstract political system they didn't participate in.
At the same time, you can observe the rise of monarchical ideas. Even Cicero said that, besides the Roman mixed constitution, the monarchy is the best constitution. Unsurprisingly, people become more willing to accept a single ruler if they can't rule themselves. If soldiers fight in Gaul for tens year under a human, charismatic general who give them their share of booty; if they never participated in elections in their life; if they know that most senators despise them - why should they side with the Senate against Caesar? It's more than obvious that the Republic had lost a lot of legitimacy in the eyes of simple citizens.
That's why the people of Rome didn't protest when Tiberius "transferred" the elections from the people's assemblies to the Senate, as Tacitus writes.
Now how can the Roman Republic reform? Honestly, I don't know. None of the reforms undertaken by Pompeius in 52 BCE (for example the law providing for a period of five years between a magistrature and a promagistrature) had the potential to stop the vicious circle I descriped above.