It will never happen because MJD is likely asking for more then Wallace. Also, it just doesn't "seem" like a Steeler move. That said, we've talked about this lately and I would love it if was possible. It would be great to have him, BIG improvement to the running game and he picks up the blitz well. Not to mention the fact that opposing teams would have to put 8 guys in the box (with MJD running and Colon/Pouncey/DD pulling there would be no other way to stop him) and that would allow Ben and all our receiving weapons to cut them up. It's not going to happen though, and even though I think it would be a good trade for us, I'm also very happy with Wallace and our receiving corps, and I'm pretty confident in Redman IF his injury doesn't linger.
As for the comments about taking him over Mendy, it seems like a no brainer, but Mendy has not had a chance to actually show what he can do due to a poor O line and an even worse O Coordinator. Based on that you can easily make the argument that MJD is a lot better as he has not had ANY help and he was still the best in the league, while Mendy at least had the passing game to take some light off of him and all he did was dance behind the line. BUT, I've re-watched every game from the last 3 years and IMO Mendy has the potential to be a top back (Ray Rice like). He runs hard, has home run capability, and seems to have great all around talent, he just can't do it all on his own. I would still take MJD, and I still stand by what I said last season when I said that Redman was a better back FOR US behind last years O line, but we haven't seen the best of Mendy yet.