At one point it is mentioned that while growing up in Hailsham, students were never really sat down and told what was planned for their futures, but instead gradually allowed to hear bits and pieces of information a little at a time, or told about things when they were just slightly too young to understand, so that by the time they did understand it, the knowledge itself was nothing new. The author uses the exact same technique to tell the audience what would happen; there's never a Reveal, he just gradually allows us to guess, through scenes from Kathy's adult life, what's really happening. When it's finally spelled out for us, it doesn't come as a shock, despite how horrible it is. It's simply a confirmation of what we already pretty much knew - exactly as it was for the kids.
Religion Is Right: Literally right or not, does it really matter all that much? The belief Ruth and other clones cling onto, that true love will give you permission to live for a few more years, is technically secular in nature. But it also fill two other functions. One as a cry to the creators, a plea for them to recognise that the clones do have some inherent value, not merely a instrumental value as spare parts. Second, as a matter of how Ruth rewrote the story of her life, allowing herself to die with at least a shred of dignity. By choosing to believe in the myth and choosing to give up her potential spot with Tommy for Kathy, she gets to die as a martyr rather then a mere animal being taken to the slaughter. So while she failed to save herself or anyone else from death, she at least managed to save them from the dehumanization. Real Life religion can do the same thing, no matter whether the religion is technically true or not.
When Tommy and Kathy are spending time together before his fourth donation Tommy mentions a theory that circulates among the donors that the fourth donation, which is always the last, may not be as final as it seems. According to this theory, the euphemism "complete" may not actually refer to death. Rather, Tommy has a dream about being kept alive in an altered state, where he is no longer autonomous or even conscious most of the time, and is only "switched on" when they need to take more parts. Ordinary people, and even the clones themselves, are so uninformed about the whole process that it doesn't seem too far-fetched for the medical teams to be spiriting the bodies of "completed" donors away for such further farming.
