This is not the first time that Doctor Who or Steven Moffat has presented the idea of life as a construct of the mind. The body is transport. There is no soul. The afterlife only exists if we invent our own and would be nothing more than a set of memories preserved in a virtual reality. Whether to call this heaven or hell is open to interpretation. Missy is not interested in questions of religion. She only wants batteries to power the metal suits she is hiding in plain sight. She cares nothing for preserving knowledge or culture. There is a reason her victims wake up in an office. They are human resources not people. They are as valuable as the desk chair or a paperclip. The Doctor is well aware that he and Missy/the Master are not so far removed. He travels with companions to remind himself that he is not the only thing in the universe that matters.