Its not complicated. Lynch is a Top 15 talent by anyone who actually does this for a living, and RB is as big of a need as any on this team. What we have now is average, and even Mr. Average is walking out the door after this season.
While you can find "servicable" RB's like Julius Jones in the mid rounds, you generally have to take elite RB's high in the draft. Star runningbacks touch the ball 25-30 times a game in carries and receptions combined. Star WR's only touch the ball 5-6 times a game. The only other position on the offense more important than RB is QB, because they touch the ball every snap. You only get a finite amount of offensive snaps a game. In other words, every single offensive snap is a precious and rare commodity that every team should take advantage of because you only get so many. For the life of me, I'll never ever understand why some people are content with giving away HALF of our finite, rare, precious offensive snaps per game handing them off to a "servicable" RB. It's playing with one arm behind your back.