> If everybody has a college degree, then those skills aren’t rare and you won’t get paid highly for them. Salary is about how hard you are to replace, not how good/qualified you are.

I disagree. Your model only works under the assumption that the economy can only have so many jobs. If the US graduated 10x as many engineers from 1990 until now, the economy would be much larger than it is today. Which means that salaries may even be higher because despite a 10x increase in workforce, the demand for them may have increase by 15x.

This is an easily observable situation. Regions with concentrations of talent tend to have much higher salaries for such people than regions where that talent is scarce.