and how would you stop a hijacker taking the key from the crew, or obtaining said keys from the black market. You are solving one minor problem by introducing a whole new problem
Hand-wringing about "what ifs" when a Germanwings F/O took out a whole plane full of people due to this "feature".
"...in case the person with the key has bad intentions.
You're forgetting the case where one of the flight crew has bad intentions,
which has already happened!