The tricky part for football is that the physical development of athletes just isn't there for guys to go pro out of high school.
Look at the top rated prospects each year. How many of them could go immediately from high school to the pros? There has to be some sort of developmental league in between.
However, since college football exists, the NFL has zero motivation to create a developmental league. I agree that it would be the best system for the college game, if players had the option to just go pro out of high school and at least get paid minor league rates before developing to the NFL level. However, that'll never happen, because it would require the NFL to stand up a league that it currently doesn't need.
Sure, there are talents that the NFL misses out on because they can't cut it academically and end up in Jucos or flunking out of colleges before they hit the draft. If they miss on 50-100 players every year that they could've had, it doesn't hurt the overall quality of the product, certainly not enough for them to justify setting up an entire league in order to make sure they capture all those talents.
How much is the NFL hurting because Nick Brassell was incapable of passing college courses? A guy like that might have made it had he been able to go play for some minor league team for a few years where school wasn't required. The NFL isn't hurting though. They've got plenty of other talented guys to be able to sign to fill rosters.
My idea of the perfect system would be to allow a certain number of athletes each year to attend as non-students. Maybe allow 10 roster spots per team. Those guys can take courses set up to teach them about personal finance, business investments, and some of the potential pitfalls of being a professional athlete. Those courses could be available to other athletes too. I certainly think there would be value in that, but just allow them to be non-degree seeking members of the team.
Dismiss the farce of the student athlete, and at least allow those that have no business or desire to be a student athlete just to be an athlete. Pay the players a portion of what the programs earn. Maybe standardize it across all schools with cost of living adjustments. You could even create a luxury tax type system where all those disbursements come from the NCAA, not the schools, and the school has to pay into the pot based on certain measures to determine their revenue level. That way an Alabama ends up on a level playing field with a Vanderbilt. Then you're good to go.