I think the crucial time period is the decade before the Social War. If the reforms of Drusus had been pushed through- even if Drusus died shortly afterwards once his personal gains from the law had been dsicovered- then the incredibly damaging Social War might have been avoided. Moreover, with those reforms the Republic would have been much stronger constitutionally. It would still have needed to reassert control of the military, but so long as the legions aren't fighting on Italian soil that's easier to do.