It's an academic credential to show an ability to do research, and to present that research, in an academically sound and rigorous method.

I would be quite surprised if the overflow of PhD holders in science spent that extra decade of double hours and crappy compensation just to say they did it, and expect that percentage is in reality negligible.

If more people are granted PhDs per year that want to do research than are needed, then there are too many produced. It's really that simple.

It's not. If it were, it wouldn't be called "training".