I can't prove anything, but I can provide an example of where genetics seems to have been the limiting factor. My parents played sports in high school but were both bench warmers and neither demonstrated any significant talent. I started running as a freshman in college. I went to a brand new high school with only freshmen for the first year. In the first week of practice my coach told the team that we would (1) qualify for state that year, (2) we would win state by the time we graduated, and (3) we would qualify for NXN by the time we graduated. For a skinny freshman who had never run more than a couple miles in a row that seemed awfully ambitious (we did 1, got 3rd and 2nd at state junior and senior year, and didn't qualify for NXN). However those goals fueled my training. I ran 20:13 for 5K my freshman year. By the time I was a senior I ran 16:25. I improved past a lot of my more talented teammates by working harder and being more diligent, but as most of you know, 16:25 isn't super impressive for high school boys. I broke 16 in college but never broke 15 though I only had one good track season due to injury and COVID. Ran 25:35 for 8K. Again, nothing super impressive but decent for a mediocre team. I have watched many of my teammates be as fast or faster than me on less training (and no I wasn't overtraining with my measly 50-60 mpw). I enjoy racing and competing but I've accepted the fact that I will never be truly competitive outside of community road races.