If the Cowboys can't replace Murray's production then they will be go back to being a .500 team. Before this year they were average because they were one dimensional on offense and couldn't stop anyone on defense. Having a healthy Murray for most of the season and committing to the running game allowed their passing game to be more effective and it kept their defense off the field.
The running game may be a lost art and running backs are much more expendable nowadays, but it's still important to have a great running game. 5 of the top 10 rushing teams last year made the playoffs and 8 of the top 10 had a .500 record or better (almost 9 of 10 as Carolina went 7-8-1). On the flip side 5 of the top 10 passing teams also made the playoffs and 7 of 10 had a .500 record or better.
As far as Suh goes he is right up there with JJ Watt as the most dominant defender in the NFL and he's definitely the best DT. He draws a double team nearly every play and makes everyone on the defense better. If you really watch him play he has a freakish combo of size, speed, and ability. Playing DT and always being double teamed doesn't allow him to put up huge sack numbers and he isn't a flashy player, but whoever gets him will be getting greatness.