The Republic died gradually like a cancer.

 As liberty and intelligence have increased the people have more and more revolted against the theological dogmas that contradict common sense and wound the tenderest sensibilities of the soul. 

Catharine Beecher

It is interesting that you mention that historians find a time to declare the death of the Republic and the birth of an Empire.  Do you think the Republic died suddenly or gradually?  Like a person who dies, these things ususally take time.  Perhaps a cancer has been working for some time and it finally kills the person.  It is the same with a civilization.  Do you think the largeness of the Roman civilization is really eventually what killed it?  Like a child who is too big for his breeches, the Roman Empire was simply expanding and needed more and better administration.  Perhaps dictators took advantage of the situation.  What are your thoughts?

The Republic died gradually  like a cancer.

Did the largeness of the Roman civilization eventually kill it?  No.  If expansion is the cancer that kills civil society then why didn't Rome fall after the territorial gains after the war with the Latins, Voscians, Samnites, Carthaginians, or Macedonians?

Rome fell as all great states fall because of moral degradation.  With that in mind, consider the following quotes in this context:

“For the hand that rocks the cradle - Is the hand that rules the world”

William Ross Wallace

We tend to think of Sempronia as a virtuous woman - the mother of the Gracchi rather than the wife of Sempronius.  If we judge a tree by its fruit, what does that say of this tree of Sempronia?