Monarchies last, but dynasties don’t.
“Consider the Targaryen dynasty. They lasted 283 years on the Iron Throne, with 17 rules, for an average of 17 years of reign per ruler. That is incredibly implausible, even with fire-breathing dragons. Consider the Roman Empire, a similarly vast political entity: no dynasty ever lasted more than 15 rulers, and no direct descendant dynasty lasted more than eight rulers. The longest-lived dynasty were either the Julio-Claudians or the Nerva-Antonines, both of whom used adoption extensively, neither of whom lasted even 100 years. But their average reigns were 19 and 14 years, so that makes 17-year-average sound reasonable.”
https://medium.com/migration-issues/westeros-is-poorly-designed-3b01cf5cdcaf