On the contrary, I would dispute both the significance and the very existence of anything we might term "Marian Reforms." I feel quite confident in saying that most, if not all, scholars of the Republic agree. The "mercenary armies" of Scullard and his contemporaries do not pass careful scrutiny. The sources are lacking for it, and even if it did occur we find little trace of its importance in the political disturbances of the first century.

The reasons for the loss of political stability in the first century and the eventual end--I'm not sure we can call it a "fall"--of the Republic are, perhaps now more than any time in the past half century or so, hotly debated. Examining the degree to which Republican politics was in fact about "issues" and not simply the apolitical competition of the aristocracy, without reference to ideology or actual issues "on the ground," is a relatively new line of inquiry. Still, already it's become clear that at the very least the assumption that political factors from outside the aristocracy played very little role in the political disturbances at the end of the Republic except to support or oppose particular individuals seems to need serious complication. At worst we can discard it entirely. The search for a "Golden BB" here is not especially productive--in any case that's not how this sort of thing really works.