An open space running back, as you refer to it is one who gets the ball in open space. Such a thing does not exist --- all running backs get the ball in traffic. An open space RB is, in fact, a wide receiver --- who gets the ball in open space. In this, I agree with you --- Lucky because he is average in traffic but good in space, and because he has vision in space (but not in traffic), and because he has good hands and speed (but not overly good power, quickness, ability to follow blocks, balance, or cutback ability) is better suited as a WR than a RB.
how is a guy that rushes for 1000 yards better suited for WR??