Restarting humanity would take a few hundred people. Putting aside the need for resources, the scale of the cities and their distance between them means that the chances of getting enough child-bearing stock all together to begin with is low. The only hope humanity has in SSR's setting is for the other cities to not be similarly dead, at most just recoiled from conflict and sequestered.
Besides, it's comforting in its own sense. Purpose and meaning are the bread and butter of humanity. The most assuring thing you can tell the last man on earth is that he gets to leave the mark of lasting longer than anyone else, even if there'll be no one to recognize it (though most people seem to have this innate hope that the next race to come from the goo or even visit the planet will get to see the last marks etched in stone)