Terrorists would probably have an easier time exploiting this button, which isn’t exactly safe either, than update servers. Or not, but it does introduce yet another attack vector, or technology that could have a bug that makes it trigger by accident.
On a general aviation aircraft this would be the power switch for the ECM. Super easy to add a kill switch, if the car manufacturer chose to do so. No idea what a self driving car would do if it lost its ECM and related controllers.