You'll have to modify the natural key if later it is decided to add rankCategory ("best overall", "best ambience", "best value" etc) via rankCategoryId foreign

You'll have to modify the natural key if later it is decided to add rankCategory ("best overall", "best ambience", "best value" etc) via rankCategoryId foreign key to the RestaurantRank table because a given restaurant can have the same rank in the same year under multiple categories. A synthetic key avoids you having to mess with the key to handle such cases.