The main reason you want your boat to have a bit of weatherhelm is because it makes the boat go faster upwind. And just because you have slight weatherhelm at optimal upwind trim does not mean the boat will fall into irons if the helmsman goes away.

There are devices to shut off the autopilot for a MOB alarm, but that won't stop the boat. And even if they did put the boat head to wind or heave to, you can't swim fast enough to catch even a drifting sailboat because it has so much more windage than you.