Can untreated severe secondary osteoporosis of the spine, without fracture cause death? and how?

Perhaps indirectly: There is some evidence that multiple compression fractures in the midback (thoracic area) can interfere with lung function and reduce lifespan.

OP mortality: 859000 medicare patients at least 65 included in this review. 4 year survival after fracture 50% without surgery (kyphoplasty or vetebroplasty). With surgery, 24-44% lower mortality over 4 years. Secondary causes of death more likely. Only 28% in one study died as a direct result of fracture. Lung, stroke, etc more likely. Journal of bone and mineral research, vol. 26, no. 7, july 2011.