You can't go lower than minimum wage. Those making minimum wage would benefit the most.
Companies still have to keep competitive salaries to maintain a workforce. If they lower their salaries they risk losing employees who can choose to go somewhere that doesn't lower salaries.
It won't cost employers anything to keep the same salary. This money would presumably be coming from reallocations of taxes that are currently being collected. For the employer it's like getting a free raise for their employees.