The key to locking the doors is not "I hear you knocking, but you can't come in," it is denying communication to the smelly chap in the cabin with unorthodox views on the conduct of the flight.
If there is no way for the cabin to communicate with the flight deck (after a panic button is pressed) the smelly chap can't really stop you from landing the aircraft where you want it to.
Fine, he can still blow it up or slice up the SLF, but this is not as much fun to him, so he will lose interest and blow himself up on a bus instead.