The degree is worth what it can "buy". If a generation ago the degree got you to the 20th percentile earning bracket, but today it gets you to the 40th, then that degree is "buying" you less earning power overall.

Concrete example: a generation ago you could get your foot in the door at a bank with a high school diploma as a teller. Today? You need a 4-year degree in finance. Further, if you walked into that same bank with that same 4-year degree a generation ago, you'll probably be much further up the ladder than teller.