Absolutely spot on Fiddy. The real elephant in the room regarding this issue is the greed displayed by College and University Administrators. When the money became so easy for students to get their hands on, tuition costs went through the roof.
It's a crime, and Sander's plan will only exacerbate the corrupted atmosphere.
There were many things at play that increased tuition costs. Student loan access and sheer volume, decrease in state funding of university systems (depending on the state, by the way), administrative expansion and duplication, university building construction, and so on.
I think we should be careful not to put most of the blame on more students wanting education, because in many ways that blames people for wanting to increase their station in life and doesn't really address the other problems involved.
What we can do now is tighten the excesses.
Middle and upper level offices have expanded, while faculty and students alike cannot make sense for what they do to aid research, education, or the vitality of the university. Administrative salaries have significantly increased while teaching salaries have absolutely plummeted to Walmart part-time level salaries. Construction costs in the university have gone up, sometimes filling the buildings with expensive, pretty features, that could have been seen as completely unnecessary. To them a nice looking building with all of the frills is an advertising benefit. What it truly does for the student is far less certain.
Students in traditional universities are frequently required to spend an incredible amount of money in dorm living and food plans for at least a year. The quality of each dorm truly varies, but the costs are often impressive. It's not as if a student can split the cost for their measly accommodations either. Each student gets to fork up an individual fare that goes straight to the university.
While costs have gone up, tenure and a stable faculty have gone by the wayside. While the traditional university has prided itself on being research institutions rather than teaching, it has significantly decreased the ability for persons in academia to do so.
Students no longer are being taught by people with some semblance of job security or enough time to actually improve the mentorship of their pupils. When teachers are split between many campuses, making little wages, and having no offices, students are being taught by teachers that are burnt out and have little ability to devote their attention toward traditional tutelage of students. They make little money and are too damn tired to care anymore. As you progress through a program, a student ought to have a healthy relationship with their mentors and improve their crafts so they can exit school with their skills developed and their social networks expanded. If your professors increasingly seem distant, you lose a vital component to higher education: apprenticeship.
Meanwhile states across the country have viewed higher education funding with increased indifference. Funding has plummeted at about the same time that tuition costs have skyrocketed. Without the support from one direction, schools have resorted to getting the funds elsewhere: the federal government, private banks whereby the final payers become students and their families. This isn't true in all states. Mine has been a stalwart supporter of the university system. Even in the midst of corruption from the universities with regard to their administrative bloat, my state has funded them as they desired. Nevertheless, the issue is to be found throughout the country.
What will taxing stocks truly achieve in the midst of such structural problems? Nothing positive, in my estimation. It may, as you have said, incentivize further fiscal licentiousness.