In either Speed Trap, or the CTFS, or both, Charlie talks about the sprint position and how important it is, as well as how difficult it is to maintain for 100m, or close to 100m, especially for non-world-class athletes (I’m paraphrasing, but that’s how I remember it).
And that’s one of the many reasons for short to long. Since the athlete can’t maintain the exactly-correct position for 100m at the beginning of the training process, workouts are done over shorter distances, and gradually extended to 100m sprints.
However, SE is done over much longer distances, such as 200m to 300m. Is it not done until the athlete can maintain the correct position for all of the SE? Or is lower technical quality OK? Or is SE done at a bit lower intensity so the sprint position can be held? Or what?
There has to be some compromise with the position over longer SE. In fact, since even top athletes have trouble maintaining this posture through to the finish of the 100m, it is inevitable that there will be some deterioration over longer runs.