This sounds like a pay optimisation for high performers. After all, good people aren't leaving for Mars but to an employer that pays them fairly(since they know what's their fair pay).

Besides, salary is not the only thing that keeps people or makes them leave. High quality management, pleasant work environment, office location and many other things are also part of the decision making.

In fact, hiding salaries is a welfare scheme for low performing businesses. It will eventually degrade the quality of the society since it's not meritocratic and only sustainable due to information disparity between employer and employees.

The margins of a company shouldn't be coming from the employees tricked into working for less than they can get.