I don't understand this line of thinking at all. If you apply this rationale to DeMarco, then you can also say Dez gets all his catches because the QB has time to throw the ball to him. Or Romo is only good because he has time to throw. Or if the guys weren't blocking inside then Jason Witten wouldn't be catching passes, he would be blocking. Therefore, we really don't need to pay for ANY skill positions on offense because our line is so dominant. We all know that this sort of idea is a fallacy, but for some reason, it seems to stick when it pertains to DeMarco.
Thinking running backs are plug and play is pretty absurd to me. Everyone thought Troy Hambrick was, at one time, considered to be the next best thing to sliced bread, before they made him the starter. You might be able to get a guy who is able to replace DeMarco in the draft, or free agency, but in my estimation there aren't that many that can do what he does and none of them come cheaply, or without risk.
If this team was in rebuilding mode, I think your line of thinking would have more validity. With Romo's window of opportunity nearing the latter stages, that is personally a chance that I would not take, unless it was cost prohibitive.