"But College Graduates Earn More"

One of the arguments made by proponents of near-universal college attendance is that college graduates earn more over their lifetimes than do people who did not attend college. That's true based upon historical earning figures and averages, but even during peak years of income for college graduates you will find examples of college graduates who, thanks to their choice of degree or academic performance, have limited opportunity for income, as well as plenty of examples of people who did not attend or who dropped out of college and ended up making a great deal of money. A similar analysis could also be made of specific college programs, and would no doubt demonstrate that graduates within certain fields of study (e.g., engineering, architecture, math, computer science) on average earn significantly more money than those from other fields (e.g., social work, communications, political science, philosophy, art).

It's also important to recall that the opportunistic proponents of "college for all" are pushing for-profit colleges, paid for by student loan, that often produce both high debt and a certification that has little to no "real world" value. With an abundance of college graduates to choose from, a college degree is also becoming a de facto requirement for a broad array of jobs that were formerly open to high school graduates. The issue isn't whether a degree is necessary to perform the job - it's that the requirement of a college degree is a quick and easy way to shorten the stack of applicants, and suggests also that college graduates are no more expensive to hire than their peers who lack degrees.

If you want to argue that "College is a voyage of discovery, and everybody who wants the opportunity for that voyage should be able to take it," fair enough - but you should be honest that the subject has shifted away from maximizing income, and also that there is a price to be paid - tuition costs, student loan debts, opportunity costs, etc. - in taking four or more years to pursue that voyage of self-discovery. Not everybody can afford the luxury of a college education. If you push universal college on this basis, you also need to consider how it affects other students - having significant numbers of unqualified or disinterested students in their classes can detract significantly from the college experience. Like it or not, college subsidies are going to continue to trend downward.

If you want to make the purely economic argument then you should be looking at more than "college vs. no college". You should break your argument down so that people making the choice based upon your promise of heightened future income will have the information necessary to select a course of studies that is likely to lead to higher income. If that seems to mercenary, then you're really making a "voyage of discovery" argument. If not, then college should be presented among a full range of career options so that the prospective student can make an informed choice.