... is the role of the haggler. Necessary? Sure, but not the driving force one might think.
Unions are the cart, productivity is the horse.
Oh and state-employee unions should be abolished full stop. The state is disconnected from price signals, and the funds come from third parties. Meaning a governor will agree to take more money from joe taxpayer to pay for teachers.
If a market union's demands become too great, a firm can always decide to close shop or move to India.
If a state union's demands become too great... well, just try to "opt-out" of state programs and their concomitant taxes a la carte.