The potential to attack from Level 6 down onto passengers on Level 5 can easily be eliminated right now by simply installing the tall glass panels shown in the preview video.  Another option would be erecting a ballistic glass canopy over the screening queues.  That would then reduce the problem to preventing attack from Level 5 itself, which as you point out can be done with a focused security presence.  There should be more watchful eyes focused on the various entrances anyway, as there are many avenues into the building and plentiful opportunities to create carnage that don't even involve the checkpoint queues.  In fact, under the proposal the southern third of the terminal would remain outside security to serve non-passenger meeters/greeters.  This in itself would create a more concentrated mass of people who would be unprotected, turning the current advantage of crowd dispersal throughout the terminal's vast space into a liability.  Seems no different than the present problem.