The point is that everyone in society should contribute equally to it. You could have 100% of the population contribute a year and a half instead of 50% of the population contributing three years. The point is that no sector of society should be left out to prevent resentment. There is too much of a "me first" attitude going on which is counterproductive. For any country to survive, its citizens have to support the greater collective both physically and socially.
Iran deal opposition grows among U.S. Republicans, Reuters poll shows (Reuters)
from the article: Half the people are enough