I think I am at least a standard deviation out on this, but my college experience had a lot of very good both theoretical and practical training which served me extremely well as a grad student and is continuing to do so in my current job. While I could imagine having done it in less than four years, the idea of learning all that I did and getting the practice applying it that I did in less than two or three years is insane. While college does have a very high signaling value, it can also be very good at what it is nominally for: teaching students. Although individuals looking to get the best jobs may find that the signaling value is what matters in achieving that goal, on a society wide level, it probably makes more sense to spend effort on increasing the quality of education.
That being said, don't underestimate the other reasons that people of most classes in the US go to college, namely, the social experience both for itself and for networking later in life.
The signaling view of college holds that graduates of elite colleges earn high average salaries not because of what they learned in school but rather because top colleges select for students who have highly valued traits, the two most important probably being high IQ and strong work ethic. Since in rich countries almost every smart, hard working person attends college not going to college sends a loud negative signal to potential employers. Elite colleges, of course, are fantastically expensive signaling devices.
Although I teach at an elite college I have a proposal for an alternate much less expensive and probably even more accurate signaling mechanism. An organization could have a one month program which only admits those who get a high score on the SATs or some other intelligence test. Then the entire program would consist of spending sixteen hours a day solving by hand simple addition and subtraction problems. The point of the program would be to show that its graduates can spend a huge amount of time doing extremely boring tasks with high accuracy. Graduating from the program would signal that you had both a high IQ and strong work ethic.
If the program had a reputation for graduating valuable employees then I suspect it would become desirable to many recent high school graduates. The challenge would be for the program to initially earn its reputation. Perhaps it could accomplish this by having some well-known backers, by giving big cash grants to its first few graduates or by promising the first few graduates attractive jobs such as at the SIAI.