I'm not so certain a college degree is that great a differentiator - I'd agree most successful college students (graduated w/high GPA) probably have some study skills and techniques that might serve them well in API/Primary. Though a lot of bright successful high school graduates probably similarly have a solid start on study skills & techniques, too.
Sure we have more complex airplanes and systems than in the '50s ... there's a good reason we spend time with schematics and charts. The RAG is there to make system experts from art majors and everyone else. I don't disagree that as a group the college graduates with high GPAs tend to do slightly better, as they probably hit API with fresh and honed study skills, and I submit that a solid start in API helps throughout the rest of the pipeline so advantages go to the students w/good study habits and techniques.
API is way too late to be screening out a significant portion of student aviators, by that time the Navy (Marines and Coasties too) will have already invested a lot of time and money in training them to be officers and moving them down to P'cola for training. While there will always be attrition keeping it down a reasonable level is the point of screening folks before they even start the process.