This is a really intriguing question. I know that the current trend is to claim that “aero is everything” and that weight doesn’t really matter on flat terrain.
But I feel like weight matters somewhat more on “flat” terrain than most of us understand. I sometimes commute with a backpack with 20-25 lbs in it and I feel like the toll of constant small-ish accelerations is pretty significant, compared to when I have no backpack on. And I also question the impacts of continuously slightly rolling terrain that we describe as flat, because it mostly is, but that in reality is constantly undulating.
I realize that I am less aero with my backpack. And I understand that a heavier rider will decelerate less than lighter riders, generally. My comment regards the toll of constant small accelerations that are typical on a flat-ish road. Aero is the most critical factor on flat-ish (do most of us ever ride truly flat terrain? Isn’t it almost always somewhat rolling?) terrain, but I believe that weight matters more than the most current thinking accounts for.
Does the difference between a 16-pound bike and a 17-pound bike make a big difference on flat-ish terrain? No, probably not in a way that matters to most race outcomes. But I’m also thinking, wattage held at a constant, that the difference between a 200-pound rider + bike + kit is a big disadvantage compared to a 175-pound rider + bike + kit once all the different real-world factors of flat-ish terrain are accounted for.