Lack of evidence doesn't disallow us to use logic to come to conclusions.
Otherwise, I could just say there is no evidence for a conspiracy; therefore, it is totally nonexistent.
Logically, it makes no sense for anyone to try and encourage more people to become involved in the conspiracy. It would take more money to keep them quiet and complicate their alleged system. Plus, it would barely impact how convincing a round earth is to the modern world; it's already pretty much written in stone for most of us. It would be easier to just say that space travel is dangerous and that only highly trained elites can go to space.