Final answer:
The Julio-Claudian dynasty failed due to a mix of internal strife, problematic succession, and the incompetence or insanity of some rulers, which led to assassinations and coups. The Senate was largely powerless to restore the Republic, leading to a tumultuous period that ended with the rise of the Flavian dynasty.
Step-by-step explanation:
The Julio-Claudian line failed due to a variety of internal conflicts and issues with succession. After Augustus' death, his stepson Tiberius became emperor, ushering in an era of imperial rule where the Roman Senate had largely lost its power to the emperor. The possibility of a republic restoration was distant, as Rome was wealthy and powerful under the Caesars, and only idealistic senators entertained such dreams.
However, the system of hereditary succession was flawed, with risks that a ruler might prove incompetent or, as in the case of Caligula, become insane. This led to his assassination by the Praetorian Guard, who then installed his uncle Claudius. The rule of Claudius was a period of expansion and consolidation, but the lineage did not lead to stability as the succession of emperors faced various challenges.
Ultimately, the Julio-Claudian dynasty came to an end with Nero, whose unpopularity and the revolt against him marked the culmination of the dynasty's troubles. Following Nero’s death, Vespasian emerged from civil war to start the Flavian dynasty, which followed some of Julius Caesar's precedents but did not descend from the Julio-Claudian line. The assassinations, power grabs, and internal strife within and without the Senate resulted in the dynasty's collapse.