Employees may benefit from open salaries, but it is not at all clear they benefit from fixed compensation. By removing any option to bargain, wages can be kept suppressed by appeal to the system that defines them; collective action would be needed to negotiate a rise to everyone's salaries, and this is unappetizing to most. Currently, good negotiators can help to steadily float the compensation of all employees by raising the compensation percentiles across the board (good negotiators are likely spread across companies that compensate at different levels of the spectrum, based on field, ability etc), which are typically used by companies when calculating their opening offers to candidates.

So, open salaries are a good thing because they increase the number of good negotiators. Fixed salaries are not, because it eliminates all negotiators.