Or worse scenario: an employer, knowing about potential consequences, would be more interested in lowering salaries of high payed workers to avoid low payed ones leaving the job or losing productivity ("Why should I try harder?").
Does not compute - Why would the high earner stick around if their salary was lower?
If transparency was applied everywhere, the new employer would have the same incentive to keep it lower? They would still earn more but despite the transparency, not because of it.