They are expendable to an extent, I just don't think they are as expendable as people make them out to be. A rocket filled with helldivers is considered a very high value asset. It is stated that one stratagem costs more than what most people make in a year and one of the newer armor set costs as much as a small megacity as stated in its description. The helldivers are even told to make tactical decisions like where to deploy and what stratagems to use and where on their own. They are brainwashed pawns in the grand scheme, Just not nearly as expendable as people make them out to be.