If historians have a favorite pastime, it is speculating what might have happened if something different occurred in history. That being said, it is not the historian's role to speculate but to report the facts as accurately as possible. Here is what historians know about Tiberius Gracchus and his land reform proposal.
Tiberius was not alone in making what most in the Roman Senate thought to be a radical proposal. Tiberius's brother, Gaius, served with him in the Senate and was an ardent supporter of his brother's suggestion to redistribute property awarded to wealthy landowners from the spoils of war to lower class citizens. The brothers represented plebs or the common people in Rome's society. As you might suspect, the wealthy landowners were not keen on the populist proposal of land redistribution. In Roman culture, property equated to wealth, status, political power, and membership in the Roman Empire's ruling class.
Once the Senate rejected the idea, the brothers agitated the Senate's conservative members by promoting the land distribution scheme directly to the less affluent people to win popular support. Some historians believe that Tiberius so alienated the other Senate members that they had him murdered. Gaius suffered a similar fate. Though the Senate was not directly responsible, Gaius was killed by one of his slaves. Many historians believe the Senate probably paid the slave to murder Gaius and put an end to what they thought was a nonsensical idea. Though not entirely responsible for the decline of the Roman Empire, land reform did play a part in Rome's declining power and in the eventual chaos that ensued when the empire began breaking apart.
Are structural reforms more critical than reformers in history? One answer is that strong figures lead movements. For movements to succeed, timing is equally important, as is changing popular or cultural sentiments. By the time of the brothers' land reform proposals, the cultural views regarding the equal sharing of wealth, benefit, and power were shifting. Tiberius and his brother were not the first to recognize this trend. Judging by the Senate's response (murdering both brothers!), the senators were completely aware of the imminent dangers an economic land revolution would bring to their vaunted positions of power.
The confluence of ideas, popular support, shifting tide of cultural belief, and strong leaders are of greater importance than structural reform. Structural reforms are nearly always the result of rearranging societal belief systems and are nearly always articulated by an individual's vision. They are inseparable components of historical events. Once a structure such as a government loses popular support, it is in danger of being replaced by new leadership and system. By this view, one does not exist without the other, but the leadership makes the change happen.