If every trucker in the US quit tomorrow, they would be replaced in a week or so, by similarly paid drivers. Not enough truck licenses? The government would pass emergency legislation, allowing anyone who has held a car license for 3 years (with no major traffic offenses) to drive a truck for the next year or so. It might cost a few thousand in sign-on bonuses, and there would be some horrific accidents caused by inexperienced drivers, but the supply of replacement drivers is there.

If every programmer in the US quit, they wouldn't be replaced nearly as quickly.