If the NFL had to allow its teams to draft high school athletes (and they do not because they collectively bargained for the three years out of high school rule), they absolutely would draft high school athletes, especially QBs. In this scenario, they obviously wouldn't play in the NFL right away (although great WRs might play as rookies out of HS), but the NFL would develop a system to train them up and then play them when they are ready.
Do people really think that not a single team out of the 32 franchises would spend one of their seven rounds of draft picks on Arch Manning out of high school? They would do it just to take the chance on the upside, because they are unlikely to get that chance if he goes to college, unless they have the number one draft pick.
Like in the NBA, eventually the teams will start drafting more and more younger players just to stash them for the upside and to keep them away from their competitors.
Of course, this will never happen, because the NFL prefers to forbid it so that the players can be developed by the colleges and they do not have to waste that money. The NFLPA also has no incentive to change it, because they would have to give up something in exchange and none of the NFLPA players are in high school or college, so fuck them kids.