Roman Historians-What would have happened had Julius Caesar had not seized power and became a dictator?

With the caveat that I'm someone interested in history rather than a historian, I think the genie was already out of the bottle.

After the Gracchi, it both became apparent that the cohesion of past years that kept the aristocratic squabbles from spilling into public life was gone, and that populist politics and rabble-rousing have entered the political life. Marius demonstrated that there was a strong political force in the society of people who had wealth and capability but felt disenfranchised by the political system. The Social War, Sulla's march on Rome, his reactionary attempt to stuff the genie into the bottle... To me these are all indications that the political system was crumbling.

It's not surprising, really, seeing as most of the institutions were designed hundreds of years before. They needed to reform the system to suit this new empire that they had instead of working with institutions that were designed to run a single city. Seeing how they handled the Social War - assassination of Drusus, and how many lives it took before they decided to give Italians citizenship, I think the pushback from the conservatives against any kind of reform that would diminish their traditional privileges would have been too much for reform from within the political system, and the political system itself had so many potential for vetoes which made it difficult to push through change, that it's highly unlikely that it would have happened while the clique that was in power at the time of Caesar's revolt was in the Senate.

The final piece of the puzzle is the Marian legions and to whom they ultimately were loyal to. It certainly wasn't the Senate. As soon as there was a general in the field that was in odds with the Senate the potential was there to do what Sulla did. After all, once you are dictator you can legitimize the entire operation, he showed that much as well.