"Need" is a strange way to think of it. The country doesn't
need any particular amount of labor; it's instead individuals that
need money, derived from working, to live. We don't need to have everyone employed; we just need whatever employment rate is required to ensure that everyone has money in their pockets.
The problem, I expect, simply comes down to the size of family units (by which I mean "clusters of people willing to internally redistribute income.") The fewer of those you have, the more individuals need to either be employed for themselves, or get income transfers from the government, to be able to eat/live. And I have the sense that there are far more people living on their own than there were in the 50s.