I don't believe that the economic value of a degree is it's scarcity. I believe its value is the material that the graduate is exposed to, and the relevance of that material to the chosen profession.
Most of the value of the degree
is unfortunately scarcity value. You can see that from the number of employers that merely require "a college degree," without reference to any specific field. As a simple example, to enlist in the (US) military as an officer
requires a four-year degree, field not relevant. Obviously, the Army doesn't
care what material you've been exposed to.
In practical terms, the college degree has become a politically and legally acceptable substitute for things like the old IQ test and various background requirements (I remember seeing some interesting ads from the 50s that demanded "varsity sports experience" for a job selling insurance or something like that). Varsity sports experience actually makes a certain amount of sense when you realize that it's a documentary proof of the fact that you are a "team player" (literally). (Well, until a gymnast or a weight lifter or other "individual" sport competitor applies.)
Basically, a college degree proves mastery of a certain field that is likely to be irrelevant for the majority of jobs people take. You can't really get a college degree in "sales," despite the fact jobs like insurance salesman and real estate broker are among the highest-employing positions in the United States. (And, yes, there's enough competition that most employers want a college degree.)
So your degree in ichthyology won't be relevant to your ability to move three-bedroom condominiums. But it will illustrate to the potential employer that you're bright enough, motivated enough, and enough of a team player and rule-follower that you managed to get through four years of college. You didn't decide that you hated your professors and simply disappear into the dorms to play computer games. You were able to figure out and follow the rules for graduation. You were able to learn material well enough to pass tests on it, and do your work close enough to deadlines to earn passing grades. A surprising number of young people can't do that (which is why graduation rates are as low as they are).
If everyone had a degree -- and the degree was legitimately earned, &c. &c., --it would mean that everyone had a work ethic. "Having a work ethic" is something employers look for precisely because not everyone seems to have one. If everyone had one, then "work ethic" would cease to be a meaningful or valuable employment discriminator....