assuming they have little education and have worked in manual labor their entire life
Customer support isn't a very education demanding job.
and what about quadriplegia?
Funnily enough Foxscotch I know a quadriplegia (well sorta quadrol, she has an arm but no fingers on that arm, rest of the limbs are amputated). She works as a psychiatrist.
and also it's sorta beside the point
should anyone have to die just because they can't contribute to others
Even people with downs can contribute to society and work certain jobs. Granted they can't live alone but contribution is still contribution.