Thank you for bringing up this point (and speaking to it personally from 20+ years ago!). I have been beating this drum for years since I've read Paula Stephan's research[1] on "How Economics Shapes Science." The overproduction of PhDs, the delaying of retirement by professors, and the addition of institutions to grant funding have been known since the 1980s, and the trend has been linear since then.
[1] http://www.amazon.com/Economics-Shapes-Science-Paula-Stephan...
That being said, the overabundance itself can also be a need for unionization after a fashion: can organized labor influence the number of new students to reverse the trend? Not to mention the general lack of increases in quality of life: wage levels of graduate students have been flat for decades (don't have a graph to point to, but it's in http://www.amazon.com/Economics-Shapes-Science-Paula-Stephan...).
The research of Paula Stephan covers the economic decisions made by US science research institutions[1]. Her hypothesis is that academia maximizes PhD student numbers while the number of tenure-track faculty positions decline. This presents an untenable situation in the long term, where a flood of researchers with PhDs will be unable to find permanent employment, and yet the demand for cheap labor filled by PhD students intensifies.
As you can imagine, this type of trend can be terrifying to some PhD students, despite the "freedom" that their research allows. A lack of job security makes one question their salary as an extension of their worth.
[1] Her latest book is an exhaustive look at the subject: http://www.amazon.com/Economics-Shapes-Science-Paula-Stephan...
(The bottom line is that the money goes to administration costs as mentioned, leading institutions to expand to stay relevant, thus building more buildings and hiring more PIs with full expectation for them to be funded via grants. Meanwhile, the average researcher's earnings are mostly unchanged.)
[1]http://www.amazon.com/Economics-Shapes-Science-Paula-Stephan...
How Economics Shapes Science https://www.amazon.com/Economics-Shapes-Science-Paula-Stepha...
It looks into key challenges of the scientific labor market.