What do you mean by closed box? None of the different areas are seperate. You can fly in normal space from planet to planet and station to planet.Do you think there is a chance that elite in the new era will be completely seamless? Stations transitions, uss's, megaships, thargoids, combat zones, planet surface with atmosphere... Will ED ever get away from this closed boxes game style?
Supercruise is there to just reduce the time it takes unless you want it to take years to get from one place in a system to another.