Jamming more people through mediocre and worse college programs to get worthless degrees is a waste of time and money.
And I agree with your statement that the problem starts earlier. Although most of our issues with primary and secondary public education can be traced to poverty and racism. So, I think we should raise taxes to address those as well :D.
In the real world it is the opposite - you assume the degree is worthless until you demonstrate the otherwise
This is also ignoring that we've greatly widened the demographic attending colleges. The reduced selectivity means that the average person is not going to be as top-notch as they were at one time, which means that the average expectation of an individual with a college degree will also go down. This will also have an aggregate depressing effect on our earning:education correlation. More important than ever will become median earnings for college graduates. The top graduates are earning vastly more than ever before which will mask the overall problem.
Who is undereducated? We managed to create a civilization that went to the moon back when the vast majority of people never went to college. Germany continues to be a leading industrial power while lagging most of Western Europe in college statistics (especially "academic" college like we have in the U.S.)