I don't see how the fact that the initial negotiation is on the gross salary or that employees see how much the company paid would change the fact that companies have control over the employment cost year on year? Even during the initial salary negotiation, I expect companies are targeting a certain employment cost, not an absolute gross salary that will never change.

For example when you're offered a 1% raise instead of a 4% raise, you still have more money numbers in your pocket each month, but in reality you have slightly less spending power (because of inflation) but you sure cost less than expected to your employer.

Put it in other term, your work doesn't have an absolute value that companies have to pay you in full before tax deduction. This is the only case where a tax reduction would directly benefit employees, but it isn't the case.