dorian argues that the bench variation doesn't have any effect on where your chest fills out and that's all genetics. if you look at arnold in his heydey, he definitely seems to have a rounder fuller "boob" chest at the bottom than on top, unlike a guy like franco who has a massive upper chest split but not much in the way of a lower chest. not sure if it's true or not. as someone who wants to build an insane franco-esque split i hope not, but it seems that way.
i think posture has a lot to do with the sagging you describe, back then (and still today) everyone focuses on chest and arms and end up getting massively overpowered anterior delts which leads to that hunched over, shoulder girdle protraction posture, which in my opinion allows the chest to sag over time as opposed to keeping your shoulders back and standing straight up which tightens and lifts it.
Like you suggest there many things that affect how one's pectorals develop and appear. In my case, bad posture plays a big role. Although I don't have deltoid development which probably leads to that hunched over posture.
I matured young in that I was a lot taller than my fellow classmates when I was as young a 12 and still in grade school. I was this tall skinny kid who hunch over because I wanted to look more like my classmates. This bad posture stuck with me long after my classmate started to surpass me in height. My maximum height being at 17 or 18 years when I was 5'11". There are probably medical reasons why I didn't continue to grow, but they were undiagnosed. After all 5'11" was taller than the average back then. However, my father was 6'5" so clearly something was wrong. My mom wasn't short either at 5'8".
I have a good sized hump at the top of my spine which I first noticed decades ago. Perhaps if I were to spend a year or two in traction, I might be able to change this some. At my age, I am not about to devote that much time to anything that isn't fun. I think I will just have to work with the bad posture because it isn't going to change much now.
Perhaps one of the reasons some people like me get into bodybuilding is to improve on our natural genetics. That being the case, I prefer to believe I can develop the pectorals of my dreams if I just work them long and hard enough, employing the right exercises. I'd prefer to hold on to this fantasy, if that is what it is, rather than give up. My center insertion is wide. I can accept that, although I would prefer a nice tight line there.