Right at the end of the play there, Alquist—who seems to be the last human on earth—rants ecstatically about the love between the robots Primus and Helena, which he seems to think will save the world and preserve life—or as he puts it, "life shall not perish!...It shall not perish! [He stretches out his hands.] Not perish!" (3.229)
Alquist seems a bit over-carbonated here, not least because it's not actually clear, logically, why life won't perish. Is it because Alquist has developed the formula for preserving life? Or is it because Primus and Helena are going to have sex and then baby robots? Is there a difference? Just because Primus and Helena are in love doesn't mean they magically have working reproductive organs, does it? Or, alternately, having the secret formula doesn't necessarily mean that Helena and Primus can gather all the materials they need for life.
But maybe, here at the end of this tale about mechanical bodies, the reader just isn't supposed to worry about the mechanics that much. Suddenly, machines and what they do or how they fit together doesn't matter. What matters is love and spiritual renewal and all that stuff you supposedly can't replicate with an industrial process. The robots are replacing the humans, not through revolution, but through the simple succession of generations. Through most of the book, robots have been seen as things humans build, like toasters. But here they become things humans create, like children. "What did you ever invent that was great when compared to that girl, to that boy, to this first couple who have discovered love?" Alquist declares (3.229). Though, again, this doesn't make much sense, since the inventors in question did in fact create that girl and that boy.
The end, then, is meant to move beyond materialism to spiritual truth: love blossoms even amidst the machines. That's meant to be a miracle, and it's presented as miraculous. And perhaps it makes sense that a miracle doesn't really make sense. Still, we feels strongly that someone out there needs to get Alquist a book about where babies come from before he gets, er, too rapturous. You need more than love to make baby robots, Alquist. At the very least, you need a working factory.