Setting aside for the moment the unlikely scenario of the cable coming loose so completely that it grounds itself on something at the battery
end, it would be like turning your old traditional 1, 2, Both switch set up to Off. The alternator
would be fried nearly instantly when the output was disconnected, so you shouldn't have a dangerous situation, just no alternator
It is good practice to secure your cable runs so that even if say the nut falls off the back of the alternator or the battery
cable somehow loosens at the battery post the cable can't actually move very far. Long, floppy runs of battery cable are not a good idea.