My theory of the Exo is that they are indeed aliens. Many people seem to assume they are/were built by humans, but for all we know the Exo could be a race of aliens that transcended biological life, becoming 100% machine. This makes your title question difficult to answer, because we do not really know what the Exo are exactly. If they are cyborgs, than theoretically they can die, because death is the permanent end of all biological function in a living organism, even if that organism has enhanced itself to be primarily or almost completely cybernetic. Now if my theory stands true, and the Exo are sentient machines, then scientifically speaking they can not die, because they are technically not a living organism. Does this make them expendable? That depends on your definition of life. Technically speaking, a machine is expendable because it can be rebuilt and repaired. But what if this machine can think and act on its own? What if it can experience regret, satisfaction, gratefulness, vengeance and values it's continued existence, just as all biological life does? I don't see why it shouldn't qualify as "living". It's a very hard question to answer, and I probably went on more of a tangent then necessary, but as a final answer I would say no. The Exo are not expendable.