There are still four layers of security: presentation of valid government ID, boarding pass, x-ray inspection of carryon luggage, and passing through a magnetometer for the presence of metal.
Admittedly, this is easier to pull off when no one else is aboard, but relocating some storage area to sidewalls (not in overhead spaces) adds a lot of cabin room and makes it less likely passengers will bash their heads on compartments above in that frantic post-landing-must-touch-my-carryon-immediately moment of choreographed (and mystifying) panic.