Good question.

Followup question: do you know the story of how the Frankish Kingdom of Charlemagne the Great got split up into Lombards, France and HRE (basically modern Germany) ??

Kings juggled the conflicting tasks of choosing an inheritor, as well as preventing their sons from rising up and... let's just say "William the Bastard-ing" everyone.

Yes, on occasion a random bastard son will take over all of Normandy and Briton.

So I don't really know what your question is. That happened, constantly. Brothers killed each other, and sometimes their fathers, in a mad game of power for damn near a thousand years.

But clearly the kings of the time thought the risks were worth the reward. A hereditary title of king means that your followers know who is charge if you died in battle. The risk of your sons deposing you was accepted, because the risks of dying for other reasons (disease, famine, war...) was greater.