Romans were terrible at maintaining dynasties
The Persians (repeatedly), Ottomans, Egyptians, Chinese and many other kingdoms large and small have managed hereditary or familial dynasties for over 100-500 years at a stretch. Perhaps the Mongol and Turkic empires aside, in many of the enduring dynasties these kings are not often killed in battle or assassinated; and when they are it's by a brother, in law or cousin.
Romans are all over the place family-wise. In the late Empire family is almost irrelevant. Over a third of the Emperors murdered, almost as many battle deaths or enemy executions, six known suicides. These statististatistics show an insanely unstable monarchy, they may have done well imperially but dynastically they compare so poorly as to be comical.