There have been a fair amount of studies performed over the years by both Social Security and other entities that have asked this question. The answer has been consistent. They get sicker and sicker and poorer and poorer. Less than ten percent ever return to any type of employment. I believe there are two main reasons for this.
The Government and Non-Profit Sector Social Services System is Deficient
The system does not have enough money and the services are, for the most part, not scientifically based. Because of this there are only a limited number of individuals they can help and help effectively to such a degree that they can help them find and retain a full-time job in the competitive workforce. Social Science research goes hand in hand with not having enough money. Compared to many other fields there is very little social science research that would help organizations better help this population.
In the Real World Competitive Workforce No One Wants to Hire Sick People
The large majority of employers don't want any unhealthy people working for them. That means disability of any kind, not just the stringent Social Security definition. One theory about the Americans with Disabilities Act (ADA) is that for employment it has actually increased discrimination. At the wiff of any type of health problems or disability this is what most hiring personnel will see -- an unhealthy person means calling in sick more, not being the fastest or being the slowest worker, less efficient, workers compensation, discrimination or other lawsuits, "special rights," more work for their supervisors, money lost for the company and on and on. I am sure there are exceptions but not a lot, especially if you are over 50. Then consider that a large percentage of employers are small businesses and they don't even have to follow the ADA. Whether its civil rights or enforcement of federal business or environment regulations by federal agencies, one thing is a given. Some will abide by those things right off the bat. Others will look for ways to legally keep doing what they've been doing.