Unemployment is by no means a safe measure of an “excess supply of people”.
Consider for a second a world where all work is done by friendly robots, and people can devote all their time to leisure.
This world has full unemployment of people in some sense, since nobody is working. However, this worlds is abundant of things created by labor. So labor is not demanded and therefore is not a scarce resource.
Even though labor is not needed, nobody needs to be sacrificed so mankind can achieve some survival objective either.
Even though this seems like a fantasyland far-fetched scenario that might never really happen, that could be the extreme case of one of the technological trends occuring in the present time. Perhaps understanding the magnified consequences of such trend in an extreme scenario could work as an effective tool of explanation for what’s going on today.
Technology gradually turns labor efforts less important for any individual to accomplish the set of basic things needed for his survival and overall dignified existence. That’s a matter of fact, by the way, not an opinion. Even considering the perceived evolution of minimal requirements for what’s considered dignified existence, that’s as hardcore a fact as it can get. People worked harder in the past, and people leisure more today.
Hence more and more people are opting not to lend their available time as labor, raising unemployment figures. They are unemployed, but they are not necessarily starving or subjected to insufferable living conditions. In many ways, most of them are better-off than many employed people from previous generations. But such unemployment is completely voluntary and a healthy result of people making their own decisions concerning the use of their time.