I believe in collecting data, reviewing it regularly, along with feedback from the athlete, and determine how to manipulate the training stress, relative to their goal events and time to the event. There is structure, and a plan, but the plan allows flexibility based on how the athlete is progressing.
How do we know how an athlete is going to be feeling in 2 weeks time? How do we know in 1 month time? Sure, we can write plans according to a "perfect build-up" or timeline, but that's not realistic with most athletes.
Funny how it's very complex, and yet very basic at the same time.