u/Deep_Requirement1384

People seem to be confused why the fifth, last generation are purestrain genestealers, when generations before them are progressivly more human.

When enough time passed that we have GSC infected humans who can pass as normal, it means that the cult has already spread its infulence far enough that they are close to rising up in rebellion. Which if succesfull will get planet consumed by tyranids or Wait to be consumed and meanwhile the cult tries to spread to other planets by transporting genestealers.

If tyranid invasion comes to planet the cult still tries to flee with few ships carrying key cult figures and genestealers.

The fifth generations are strongest battle creatures and for further infection. Thats why they spawn last.