That is somewhat of a simplistic rebuttal to a complicated subject.
There are such a thing as *scholarships* that some people get to help them pay to attend schools which they cannot personally afford. Although some of them are need based, but some are simply are merit based (a strange concept). Sometimes people can given partial scholarships or even full scholarships to attend a school (such as medical school) and not even have to pay back anything (e.g., my uncle for instance got one of those mysterious scholarships to attend medical school)...
Millions in scholarship money goes unclaimed every year (estimates vary from $100M-$200M), that that doesn't even include the estimated $2 Billion in pell-grant money that high-school seniors didn't claim by attending an accredited college w/o filling out the FAFSA form.
As you point out, not everyone is suited for a given field, so the idea that everyone should be able to afford to get a degree that they want is somewhat a strange notion (even if we need all sorts of people to get those degrees). So why not just stick with the current scholarship/grant scheme? Does it have to be *free*?
The common notion that everyone has large student loan debt is a myth. 2/3 of college students graduate with no debt. Of those with debt, the *average* amount of debt is about $10k ($7k in government backed loans), . So who has these massive $100k+ in loans? Why it's the ones that attend exclusive private institutions and advanced degrees (medical school, law school).
Even then, those $100k+ jumbos are only responsible for a small part of the default potential on the loans (most pay back w/o problem within 10 years). The biggest problem student loan problem is those that take out loans to attend private degree-mills like the now bankrupt Corinthian college group. Buying worthless degrees from worthless colleges is the big problem with student debt.
If you want to pick on medical school as one that "should be free", the biggest problem medical practice sees is the *underused* degree. People who have used a slot in the medical school who cease to practice full time after a 10 years is staggering (they call it the 7 year itch in the profession). When questioning doctors about this, the general issue is that even the job is high-paying, the job burns them out. Many medical school admissions people will tell you many go into the profession because of high pay and social status, but fewer go in as their *calling*. Making medical school "free" really doesn't solve this problem at all.
For the record, my uncle got his scholarship essentially for being from a small town in Wyoming. He didn't go back there to practice, but is now practicing in a different small town across the country (Rhode Island). I suspect the scholarship committee gave him the scholarship because they knew it was his *calling* to practice medicine and he was also a small town type who would probably never live in a big city. It didn't work out for them specifically, but they probably had more insight into the him than either the University of Colorado medical school admissions committee, or he had into himself at age 21. Another thing to think about before you think "free" is necessarily the best option.