It seems to me you either trust the cabin crew to have access to the flight deck, or you don't. Cabin crew bring food, coffee, tea to the pilots, which could easily be drugged or poisoned
Actually, that is a good point. And then i thought back and guess what. I do drink only bottled water on board and bring my own food. It is much healthier than the packaged stuff we get on board anyway. And then, trust is a two way street, and currently the point is that pilots are not trusted anymore.