“Staffing agencies”- sounds like a good growth opportunity for entrepreneurs. We have a friend who started a temp staff agency years ago and has become a wealthy man
What allows a union strike to work is not that the union members can walk off the job (which would otherwise be just a mass resignation). What makes the strike work is their ability to prevent (using violence and sabotage if necessary) anybody ELSE from replacing them on the job.
If all your production-line workers are from three or four staffing agencies, then you can dismiss them all, and legally bar them from coming near your place of work.
Personnel Employement Organizations (PEOs) are growing quite rapidly. You lease your employees to yourself from an agency. The agency handles all the paperwork, benefits, etc. It is expensive. However, unionization would cost more.
This would be a bonanza for them.