There are a number of factors that make the methods by which kings and their heirs or relatives inherit control and power inherently unfair, inefficient, and even potentially harmful to the people which they rule. Historically, the male heirs are the only ones who can acquire the position of a king, which makes the system sexist at its core as it suggests that female members of a family are unable to rule. In a traditional sense, a king would have executive power over all resources of the nation, which is not feasible with the current large sizes of industries, federal governments, and other branches of a nation. Additionally, such power is no longer available to royals, which makes their leadership limited, outdated, and ineffective.
The inability of a person from a lower background level to partake in leadership enforces a system of classist division as well. This kind of system would not include candidates that are qualified and exceptional, simply due to not being part of a royal family.
In the past, when monarchs had more power, this also caused incredible competitiveness within royal families. Younger children of a king who had died could be subject to violence and death from others who would want their position. Additionally, these scenarios often led to instances of kings who were still children and unfit to lead an entire country. This was the case for Henry VI of England and Alfonso XIII of Spain, who were crowned as infants and never ruled in a way that benefited their countries. This would not be appropriate in the current day, as the field of politics is incredibly complex and evolving. As such, the system of passing ruling power from the king to his heir or relatives is incredibly outdated and not practiced in its true form in the modern-day.