Three points:
1) This is basic labor vs capital economics. Our monetary policy for the past 15+ years has basically made capital free. When given the choice between building an expensive robot factory vs one that uses labor, and interest rates are 2-3%, you might as well build the robot one. So there is less demand for labor and the value of labor declines = less wage bargaining ability.
2) McDonalds corporate charges a franchise fee of 12% of revenue (not profits). Since Pappa Ronald controls the cost of inputs (fries, burgers, etc) and the prices you can charge, that basically leaves franchise owners with one option for making money: screw over their employees. Corporate McDonalds averaged $5 billion in profits over the past few years, by the way.
3) Pay your workers a decent wage and charge prices that reflect actual value. I really don't care if my cheeseburger costs $1.50 instead of 99 cents.