Bad coaches, of which there are far too many especially at the high school level, ruin runners. A lot of coaches have massive egos for no reason, overwork their athletes (too many intervals, not enough miles), and have their kids burn out mentally or physically. This was my experience in HS, and many of the coaches that I encounter at meets now seem cut from the same cloth.
I had pretty good coaching in college, but I coach myself now and I'm faster than I ever was then. I've never read any training books or anything, I just keep the workload and recovery consistent, and design workouts based on what's worked for me in the past. So the OP is probably right that a solid generic plan isn't that much different from having a good coach, but it still helps to have learned from someone at some point in the past. Of course at the elite level, you'll need that extra 1-2% that only a smart, experienced coach can provide.