One of the reasons monarchical systems are supported by the West, and this includes “presidents” who appoint their own successors, is to avoid any dramatic surprises when a ruler dies.
You will notice that crown princes usually tend to be even MORE attached to the Western powers than those whom they replace. Look at King Muhammad in Morocco, King Abdullah in Jordan, even the current king in Saudi Arabia, it could also have been said about Bashar al-Assad back in the beginning of his rule.
If the current king dies and Prince Salman takes over, he is even closer to the US than his brother. Salman is apparently already ill himself, and is not really in a state of health to rule either…so if he also dies, the next in line is Muqrin bin Abdulaziz, who is even MORE attached to the West.
You see, monarchy allows the West to prepare well in advance of a ruler’s death to establish a strong relationship with any eventual rulers scheduled to come to power; thus ensuring continuity of policy and continuity of subordination.