Replacing real walls with Theoretical walls doesn't change the fact that it's a wall from a game-design perspective.
But isn't that essentially what a leveling system is: A set progression of walls/doors, unlocking new things as you advance. If you wanted a "True" open world, why even have levels in the first place. Make everything killable at "Level 1" and ignore the rest? The whole point behind 'walls' both real and imaginary is to essentially prolong the game, since being a 'sandbox' (by the true definition of the word) one could step off the carriage (as in skyrim) and march right on to Alduin and win. That, is truly open, but does anyone really want the end bosses of these games, to be killable in the first 10 minutes of turning it on?