Liu Bei carved out a third of the country for himself during times of chaos and being chased by a much more powerful enemy. He attracted both powerful generals and talented strategists to his banner despite being at a time the weakest of the 3 Three Kingdoms rulers. Before the Battle of Red Cliff he didn't even have a permanent city/ford to his name while other warlords have already conquered entire provinces.
Liu Shan inherited everything from his father.
A ruler's achievement cannot be solely based on surviving to a ripe age.