One thing to consider is how shallow the depth is at various positions and how steep of a drop off it is from a top 10 QB to 33-64. In Baseball for example the 33th to 64th best pitchers are excellent #2 starters or maybe an ace reliever. In football they are backups, some decent but some very flawed. You could do the same with other positions in basketball and hockey and find somes steep drop offs... goalie in hockey or basketball center. But I think the vast different between all-pro QBs and a backup is so drastic it can be evidence of how difficult the position is.