Here's the logical problem with using 1st round picks on RBs:
If they are anywhere south of a perennial 1,000 yard back it's a wasted pick because the NFL is chock full of serviceable starting RBs taken in rounds 4-7.
If they play well enough to justify the 1st round pick, they want to get paid like it after 4 years and smart GMs don't pay long term contracts to RBs. So at that point the best move is to let them destroy some other team's salary cap.
In my opinion you don't take a player in the 1st round if you don't think there's at least a chance they'll sign long term after their rookie contract and spend a decade as a Bill.