Would the average American be better off if 50% of the population worked productively with them?

This is a problem not only in America but all around the world. As the world population's age go up there is going to be a number of people who are retired but have to be fed. And the burden is going to fall on the younger generation. This is partly overcome by increased productivity and partly by governments policies in support of the older generations. Also re employing the older people will ease this burden a lot. Several government all around the world have increased the retirement age. Hope this answer gives you something to think about.