Transponders are already hooked in the air ground logic and use it to set the "on ground" flag if the aircraft thinks it is on the ground. Which leads to very surprising issues if the A/G logic malfunctions and the plane takes off. Not only will there be issues with the gear, pressurization, and FMC logic, the transponder will transmit the ground flag as well which means that ATC will not correctly display it.
Anyway, as said above even transponders malfunction and sometimes need to be switched off without leaving the seat, finding the right CB and pulling it. The only thing on my small boeing flightdeck i can't switch off in any way is the ISFD, and since that contains its own battery pack it will merrily burn up without any chance to switch off electric energy...