You're missing an important part of coaching: motivation. I don't care if you know all of the science in the world. If you can't motivate your athletes and make them believe in themselves, you're finished before you've started.
Jack Daniels, for all of his lab work, was successful because his girls believed in him and believed in the training. This is what guys like Magness will never understand. The science can take you only so far.
I think all coaches should have an athletic background, but it doesn't necessarily have to be in the sport they coach. My coach in college was an All American when he ran in college and freakishly talented. He was a decent coach, but his training was solely based around what he did in school and nothing else. It never occured to him that all runners weren't like him.
The women's coach at my school never ran competitively. Instead, he played lacrosse, swam, and even played a couple years of football. He wasn't up on the latest running science, but the women's team were always national contenders because he had a basic program, individualized as necessary, and was a master motivator. He took girls who never broke 21 in high school and had them earning AA certificates by their senior year. He didn't know how to run, he had the most basic understanding of training theory and scinence, but he knew how to coach.