First, all players are not the same speed without changes to the threshold. The 50 threshold is the setting which EA suggest gives the optimal gameplay according to how want football to be played. If you want a bigger difference between the players, you lower the threshold, if you want a smaller difference, you raise the threshold. From my experience with the demo, both Cutler and Rodgers can outrun any LB with the threshold on 0.
Second, even if the threshold was 0 it would likely be a 50 yard gain anyway, because of the way the new zone defense AI doesn't really recognize with QB scrambling and moves defenders out of position, while the Goalline QB Sneak gives an acceleration warp to the QB.
While it's more difficult to preform (because you don't get the acceleration warping) the pump fake glitch also occasionally allows big QB scrambles of about 30-40 yards - even on 0 threshold.