I think the RB position is a position where you need just enough production to keep defenses on their toes. And for that reason it's a position that's easy to replace. I think the 2012 Vikings showed that's as far as you will get relying soley on your RB as your best player. Given that and the shelf life of a RB it's just not smart to throw a ton of money at a super star running back. It's much smarter to spend the money elsewhere, specifically defense if you already have pieces on offense. You can find a RB anyywhere. It's almost like I hope Lacy's ceiling isn't much higher. He's doing exactly what's needed, but if he were to start taking it to much higher levels it just wouldn't be smart to pay him big bucks over someone like Cobb or defense.
If you find a running back that becomes a stud, utilize him until he demands big money. Then plug in a new one for cheap and hope the same thing happens.
I wouldn't use the 2012 Vikings as an example -- it's not AP's fault that he was surrounded with garbage at the other positions.
Look at this years Indy team as a good example of the shoe being on the other foot: Luck has Daniel Herron and Zurlon Tipton at RB. His WRs are a rookie, a boom-or-bust guy that can't seem to catch the ball, and a 40 year old Reggie Wayne. No matter how good Mr. Luck is, do you honestly see him making the Super Bowl with that cast?
If they don't get him a solid o-line and/or weapons, they'll be a perennial one (or two) and done team. I'm not saying to start blowing first rounders on RBs left and right, but a guy like Gurley? I might actually make that move if I'm a middle round team looking to get over the hump.