In Mahayana Buddhism, do they expect everyone to become a Buddha? If so how can any Buddha help anyone to attain enlightenment?

In Mahayana Buddhism, do they expect everyone to become a Buddha? If so, how can any Buddha help anyone attain enlightenment? Isn't the whole point of becoming a Buddha to help others to attain enlightenment? It's like asking everyone in the country to become kings. Then there won't be any countrymen left to rule. To give another analogy, imagine patients refusing to follow doctor advice saying that they want to become doctors first to help cure others. Then those future patients they are hoping to help would also have to refuse their help to try to become doctors themselves. At the end of the day, no patient makes use of any doctor and no doctor cures any patient. So why become a doctor in the first place?:)