Yes, but isn't that really what every NFL team does though? They all draft on potential. Nothing is guaranteed to you at that level. But you are right, there would be some downside to this for the NFL team. In reality, simply offering and negotiating a larger contract to the players they believe are NFL ready would possibly convince him to forgoe college and jump to the league after they are drafted.
If he decided to return to college, possibly compensate that team with an equivalent draft pick the next year. I do agree that college football is the best way to develop a player for the NFL. But this wasn't about developing a player for the NFL.. It's about fixing the problem of players leaving college early to only go undrafted.
Quite honestly, kids are getting bigger, faster and better every year. I doubt there will be much off a drop off in the talent level that enters the draft overall. But I am hoping such a scenario would allow teams to get their best for their picks even if that player was a bit younger (and yes, higher risk) than some of the others still on their draft boards. Which should benefit the more experienced players overall by giving them a better understanding of the draft and what is in their best interest.