The problem with your thinking is that you are looking at it as a fan. Just because we debate what we think is the problem, doesn't mean ... well, anything. We're not privy to any of the inside thinking, coaching, etc. that goes on. However, the coaches (should) have a pretty good idea where the problem with the running game is by now. And, they've indicated it's both the line and Najee/Warren at different times. "Popcorning" So, by their reasoning, the Steelers don't need, and should not want, another RB. They say they need to fix the issues at hand .. i.e. the "popcorning".
So, I sincerely doubt that they will/would trade for a RB just to prove to themselves what they already know. Or, to satisfy fans' desire to "see if the line or the back" is the problem. (it's both).
smh