Demarco Murray has had great games in the league with our horrible oline and he too like Emmith isnt a speedster. Just grat vision, running between the tacklers, and toughness.
You undervalue the skills and talent of a true running back who isnt reggie bushing it all over the field. A great Oline goes a long way but you have to have the talent and skill to take advantage.
Having watched DeMarco Murrays every game for the past 6 years or so, he's not as fast as he was his freshman year of college, but he's still faster than Emmitt, he's a more physical back than Emmitt when finishing a run, he's better at improvising(which Emmitt never had to do), and is a better receiving back by miles.
Yes, you have to know what holes to hit as a back and have the vision, patience, but that Cowboys line opened some HUGE holes for Emmitt to average 4-5 yards a carry. If your o-line sucks, there's no way you can literally average positive yardage every play lik Emmitt did.