This statement is true (yours):
> Only a small minority of lower class people ever go to grad school.
But so is this one (mine):
> A vast majority of graduate students are from low-income backgrounds.
However, this statement is totally false (yours):
> it’s a tax break that benefits the kids of upper middle class and wealthy families.
Many graduate students, especially those in STEM fields (80 - 90%), are poor international students. Their families are already taking on generations of debt in order to give their children a mere chance to emigrate to this country.
The schools don't want to/won't pay the higher stipends required to offset the tuition waiver taxes (for the minority of graduate students who get them), and they don't want to/won't give up the massive cash infusion coming from these students (who don't have waivers) already.
Given the choice between the two, they'll choose the latter (the cash cow) and throw the former (the best and brightest) out with the bathwater, if you'll permit me to mix my metaphors. In the long run, this outcome will be bad for the schools, and bad for the businesses who hire these students once they graduate.
Sources:
https://www.insidehighered.com/news/2013/07/12/new-report-sh...
https://www.insidehighered.com/quicktakes/2017/10/11/foreign...
https://www.nytimes.com/2017/11/03/education/edlife/american...