This fact is only one of many that the SSA looks at to determine your disability status. Certainly you are going to testify that the job you worked for 20 plus years you cannot perform now due to your disability condition. However, the SSA will look to determine if you have obtained any skills that are transferable from your old job to a new and different job. If those skills transfer, there is a strong chance you will not be able to qualify for benefits.
Remember, it is your burden of proof to show that you cannot perform any job you could qualify to do in the national economy. To the SSA, it does not matter if a specific job is available for you. At a hearing the Judge will call on a VE (Vocational Evaluator) who will testify as to what jobs you could qualify to so. I have heard the VE's at a hearing state my client could be a “silverware wrapper” or a “gluer.”
These jobs sound ridiculous. You must be prepared to demonstrate why you cannot perform these jobs, even if you have never worked these jobs in your entire life.