I don't think the age limit is stupid at all. This isn't the NBA, injuries are a real thing. The first time some 18 year is forced out due to horrible injuries and without a college degree, that's the beginning of the end of the sport. The way it stands now, at least the NFL is forcing them into college. What they do with that opportunity is up to them and yes, the extremely fucked up colleges they attend.
The NFL certainly benefits from the NCAA's messed up system, but even if tomorrow they decided to throw their weight against it, the NCAA would just laugh themselves silly. They don't give a shit about the NFL and kids will never stop wanting to play ball to get their so their program isn't going anywhere.