This is an interesting take on CoG. I always “peg” the CoG to the bottom bracket, as opposed to the cockpit. The cockpit is moveable, at least to a certain extent, regardless of frame geometry (stem length, stem rise, bar reach, bar angle, spacers, etc.). However, the bottom bracket is fixed, and I view it as the “anchor point” of the bike. What I have read and learned from reputable fitters is, if your weight is not distributed properly (for your goals, body type, etc.) over the bottom bracket before adjusting the cockpit, then there will always be compromises in the fit. Adjusting cockpit can’t always compensate for the hips being too far “on top of” or behind the feet for proper balance. I always thought it was more about the hips’ relationship to the feet in space.
Think of trying to balance while bending over to reach something out in front of you (i.e., the cockpit) without being able to hinge your hips behind you as a counter balance while standing with your butt against a wall. You can only lean so far over before you will begin to lose balance and fall forward. If your saddle is too far forward, it’s kind of like doing this exercise with your butt against the wall. The hips are simply too far over the feet, providing no “counter balance.” The only way you won’t “fall over” is to bear too much weight on the handlebars, leading to hand numbness, shoulder/neck tightness, etc.
Now if you move away from the wall and allow your hips to hinge behind you, reaching further in front of you with solid balance is easy. This is because the hips are “further back” behind the feet, providing counter balance. To me, this is simulating moving the saddle back further behind the bottom bracket. I know these are not direct comparisons, but this logic always made sense to me from a balance perspective. Once the hips and feet are properly set relative to each other and the rider is balanced, the cockpit should only then be adjusted to be within a comfortable reach, right?