https://nces.ed.gov/programs/coe/indicator/cba/annual-earnin...
To relate this to your bachelor's degree example, if everyone earned a bachelor's degree in e.g. physics then the earnings premium for STEM graduates would drop if not vanish. But I expect that society's collective material prosperity would increase. I say "earned" rather than "got mailed a certificate" because getting a certificate without developing the corresponding abilities is not useful.
The point of the experience is that it allows you to produce more value with the same inputs.
To wit: nations with higher educational attainment do not see the averaging that you suggest, instead they see rather remarkable standards of living.
If it slides too far to the signal side, it becomes a bad proxy. Considering grade inflation is a thing, I wonder how much of that relationship gets eroded