Yes, I understand that if you incline a segment of fixed length to put the higher end at two different rise heights, the distance over the floor will be reduced... pythogoras and all.

But that's not what we have in dancing. The distance of comfortable, workable leg division is not constant at all. It depends on the speed the body is moving at - if you are moving fast, it's easy to divide your legs far without getting stuck in a split. If you aren't moving much at the bottom of the swing, you can't divide your legs much without getting into an awkward position.

Similary, the height of rise you can achive depends more on your speed than on your foot strength. If you are going fast at the bottom of the swing (travelling far), you will have to rise high in order to absorb that energy - fail to do so and you will stumble past the foot closure or overturn your body to a distorted position. If you get higher than your foot strength can support, your fall will be abrupt rather than a smooth transition back into travel across the floor, but that's a problem for the next half, not the first half.