This is from the Social Security Advisory Board's Aspects of Disability: Decision Making: Data and Materials. A person's chances of becoming disabled soars as he or she ages. This is the sort of thing that is obvious if you are at ground level but perhaps not so obvious if you're at 30,000 feet. This has little to do with Social Security policies. It is the unavoidable effects of aging. Any analysis of Social Security's disability programs which fails to take into account the aging of the population is inherently misleading.
This graph only goes up to age 64. Notice how steep the curve gets as it approaches the 60-64 age group. What do you think it would show if it went up to 70? Is increasing full retirement age really feasible?