Valid points, however unless and until someone comes up with a 100% foolproof way to prevent attacks (not likely) the ability to escape an attack is equally as important as preventing one.  While the current crowd on Level 5 is certainly vulnerable, they also have multiple avenues of escape with only flimsy fabric rope lines in their way.  The proposed 6th level screening areas appear from the renderings to be narrower and more linear, enclosed within hard walls.  I hate to think of what would happen were an attacker to begin wreaking havoc at the new screening entrance, and then continue mowing down passengers who would, in that configuration, only have one way (forward) to escape.  I think in that scenario what would happen is that panicking passengers would begin trampling each other as they all rushed toward the only avenue of escape and would become jammed up in the narrow corridor, making them sitting ducks as the attackers moved from back to front taking them out.