Dynasty or Meritocracy?

Devarim 17:15-20: "You shall place over yourself a king whom HaShem shall choose . . . When he sits on the throne of his kingdom, he shall write for himself a copy of this Torah . . . It shall be with him and he will read it all the days of his life in order that he learn to fear HaShem his G-d . . . so that he not become haughty over his brothers . . . in order that he have a lengthy rule over his kingdom, he and his son, in the midst of Israel."

Rashi: "'he and his son' - This teaches that if he son is fit for kingship, he has precedence over any man."

[NOTE: Rashi implies that the son of a king is only the primary candidate for kingship if he is fit for such a position. However, would the son be unfit, another would be considered as successor to the king. The Hebrew word for "fit" here is "hagun," which generally denotes refinement of character. This is the criteria by which one is considered fit to be king of Israel.]