My opinion is that hidden salaries skew the edges (wider distribution) and transparent salaries flatten it.
What I mean is, with hidden salaries, yes, the low performers get less. Company is shorting them. But also, the high performers get more.
When salaries are transparent, you start making algorithms for whats "fair", so it becomes impossible to counter offer to keep to talent. You can afford to overpay 1-10% of people, but if paying someone more means paying everyone more, then the high performer salaries go down.
Essentially what this means is that hidden salaries are good for the top 10%. 50% of people think they are in the top 10%, so they think transparent salaries will hurt them.