Even if four years of "Free!" college could be provided with no cost out-of-pocket to the tax payer it has huge costs that are not visible to its supporters.
Give him an "extra" year
Elementary school teachers will sometimes recommend that a struggling student be retained. In my experience they frame the suggestion with "I think we should give him and extra year."
That usually makes everybody feel better about the whole exercise. After all, they just increased his life expectancy by 365 days. There are not many medicines that can do that.
In fact, they did not "give him an extra year". They reduced the productive portion of his (and it is usually boys) life by one year. But is even worse than it sounds.
The time value of money
Consider the time value of money. At a six percent discount rate, one dollar earned this year is worth a dollar earned in years 33-through-40 ($8) of a person's career. That means that for a person with stagnant wages, a one year delay into the job market could equate to an eight year delay in retirement.
Four years of "Free!" college is four times worse than "giving Johnny an extra year" for 4/5ths of high school graduates. That is the 4/5ths who will go into jobs that do not require college degrees.
The half life of knowledge
Estimates for the half life of knowledge range from five years to forty-five years. The entire concept of "half life of knowledge" calls into question the entire concept of getting one big dose of "knowledge" that is supposed to last one's entire life. Even if half of what you know is still true fifteen years after you graduate from college, half of what you know is "wrong" or "obsolete".
In fields where the half life is shorter (psychology, for instance) the best case scenario is that a significant portion of what the student was taught is "wrong" the day they graduation. The worst case scenario is that their professors did not keep up with the art and taught them wrong/obsolete information.
So it is not clear that four years of college, all in one shot at the beginning of their working career, is a good investment.
So why four "Free!" years of college?
From my vantage point it can only be a veiled attempt to buy votes.