> But your value should not be measured by how much somebody else is getting paid, but by how much another company is willing to pay you.

Who are you to say that? Is the invisible hand prescriptive now? What if I value fair treatment over marginal nominal compensation? What if I don't know how much to ask for when I'm looking for work because I'm so underpaid that my perception of the market is fucked?

Same way you work out how much your house is worth, you get two or more companies in a bidding war for your time.

What is "fair" treatment? Are different levels of negotiating abilities unfair?

Your value to the company is also constrained if the company pretend they can't afford your real worth, while they pay that worth to comparable employees. You don't know the worth you should be demanding because there's no transparency. The person was being cheated by their employer here.

That's the point, you never have any grounds to demand more unless you have another job offer on the table.

When companies say they can't afford to pay you x, they mean they don't want to pay you x.

Yes, sure, perhaps the employee was being cheated, but pay transparency is not going to fix that, that just insures everybody is being cheated evenly.

The cheating happens when the profits of the company are not distributed to the people that run it, but to the people who own it.

The initial value could be measured this way. Market price discovery. Without external influences (collusion, etc.) the market will generally define price bands. The stock market is an example of this (to a degree). The job market is also an example of this.

Over time, employees show their value or not. The higher value employees should be appropriately compensated for what they deliver, e.g. more market price discovery, but for rarer assets.

The danger of transparency is that you will discover the presumed market value difference, without seeing reasoning behind those differences. Which, honestly, will result in many angry people.