If you got all this weight crushing in on you, around your chest cavity, there's no way that your chest can expand because the weight of the sand would push your chest cage in. as you let air out, they increase the pressure, so you can't open your lungs. He would get closer and closer to death with every breath. The only way they could have saved him would have been to have really dug fast and gotten the sand away from his chest cage. Unable to take a breath, Milos was dying of something called "crushed asphyxia," a medical term to describe the constriction of the lungs by compressing the torso. Unfortunately, this wouldn't happen very quickly. He'd be aware of what's going on for probably a few minutes. Actually, it'd be a pretty horrible way of dying.