It's uncomfortable to say that something in the past was "inevitable." Syme and Meier both more or less thought that if Caesar (and later Octavian) had not been the ones to establish themselves as masters of the Roman political sphere somebody else would have done so. But, say people like Gelzer or Badian (or more recently, Ungern-Sternberg, who wrote a very nice article on the subject of political legitimacy), while the political situation of the late Republic was strained this was nothing new to the Republican world, and there is no reason to suppose that the Republic would not adapt itself as it had during countless crises before.

This question is really several. In the first place, following Caesar's murder was it a foregone conclusion that the Caesarians would exact their revenge on the murderers and open civil conflict again? The answer to this is a resounding no. In the days immediately following the murder the Caesarians themselves, to say nothing of the senatorial class in general, were divided as to what should be done. The conspirators had barricaded themselves in the Capitol, and Lepidus, supported by a faction of the Caesarians that demanded immediate revenge, marched his troops into the city on March 16 and prepared for an assault on their positions. Antony, however, prevailed upon him to wait until he had negotiated with the senate, itself divided into an avenging group and a conciliatory one. Trading recognition of Caesar's legislation for a general pardon of the conspirators Antony and Cicero were able to defuse the situation rather well, and the conspirators came down from the Capitol. This may well have been where things ended, but for Antony's funerary oration on March 20 which drove the conspirators from the city. But it's not hard to imagine a situation in which Antony did not deliver such a rousing oration, in which he was fine with not being named Caesar's main heir, and in which the truce between the murderers and Caesarians remained intact.

Likewise, it's not too hard to see a situation in which Octavian was never much involved in anything. When Octavian entered Italy, having been made Caesar's heir, he had very little real status, and his securing of his inheritance was only done in the face of great opposition from Antony and with the instrumental help of Cicero. Cicero recognized Octavian's illegal private army and granted him an extraordinary proconsular magistracy after he had managed to drive Antony from the city. It was Cicero's unfortunate luck that both the consuls Hirtius and Pansa died at Mutina, leaving Octavian holding all the Republic's military forces and free to change sides and go over to Antony. But it's not too hard to see either Antony not putting up so much of a fight against Octavian or the consuls surviving Mutina. In either case Cicero might have been able to hang onto the situation long enough to restore the Republic--it had happened before, and Plutarch considered Cicero to be as close to absolute power as Antony was. In that case would the conspirators have continued to stay outside Italy, or might they have been finally pardoned once and for all?

Ultimately, though, we're forced to grapple with Syme's conclusion that the Republic was so sick that it was doomed to collapse into civil strife, Antony, Octavian, or no. There's something to be said for this conclusion, and many have defended it over the years. For one thing, it's rather hard to disprove. I can point to this or that formative event in Antony's or Octavian's career and say if that hadn't happened there's no reason to suppose this or that or the other thing might have happened. But if we consider the crisis of the Republic to be unwinnable then those specific events don't matter. A great deal of ink has been spilled on this subject, far more than would be reasonable for reddit, but it's still an open question. For what it's worth, however, we can pretty certainly say that the particular way that things played out in the days and months following Caesar's death was far from certain. Quite a lot of fortunate and unfortunate events occurred in that time that might have influenced things totally differently if they had happened otherwise