1) If your head remains relatively steady, you're going to have a hard time getting your weight outside your feet. You'd have to...
2) If you shift your hips SO MUCH (or setup in a horrible position) that your weight gets outside your feet, then you're going to have trouble with Key #2.
But we never see #2 - Sergio and Monty slide their hips back, but their weight never really gets outside of their feet. They get a little more pressure into their back foot than someone who keeps his hips relatively centered, but that's fine too. Monty and Sergio have the 5SK as well.
Just to add, each Key is suppose to be measurable, achievable and relatively simple for the average guy to identify and understand. I agree that how the pressure moves in your feet is important and can effect the head, but I can translate my head and still not have my weight move to the outside of my foot. Like Erik said, would be tough to keep the head steady and have the weight move to the outside. Then there's the educational aspect, imo the head movement is easier for the average guy to spot than what part of the foot the pressure or weight is located. Remember there are A LOT of golfers that think impact looks very much like your address position.
Even though Key #1 is Steady Head, when we teach it we don't just say, "Don't move your head", we usually share a few feels that "knock on" effects to keep the head steady. The feels may have you focus on your hips, shoulder(s), knee(s), arms, where the eyes are focused, feeling the pressure in your feet, etc.