Security can never be 100% effective. Flight crew & cc could bring down a plane in flight if they wished without bringing anything onboard but
consider coercion. Bank managers have been forced to 'assist' in bank robberies because their family was held hostage. "All you have to do Captain/Steward is put this bottle of liquid soap in the aft toilet or leave this egg sandwich in the overhead locker above row 1A." I accept, as with most movies, this is more likely with the ground cleaning/catering crew, but the risk is present.
For as long as pax restrictions apply, crew cabin luggage should be consigned to the hold, as per pax valet with only items essential for the flight operation, charts, calculator etc allowed in the cabin after airside security scrutiny. If a mobile phone is essential, an approved mobile phone could be collected from airside security. Hot/cold drinks should be obtained from the galley as should any special diet meals/snacks.
If you think about it, these meaures would protect crews from coercion but as I said Security measures can never be 100%