How to deal with crushing overpopulation.  And old sci-fi concept, perhaps one of the most famous is the Star Trek episode "The Mark Of Gideon" or, perhaps, the film "Logan's Run."  Or, geez, maybe "Soylent Green!"  I think this ending falls under what TV Tropes labels as the "Outer Limits Twist:"  not especially ironic, but sadly cruel.  Science fiction drama, time travel.