Risk analysis needed
It's a risk analysis problem. What is the risk from an ACARS system which cannot be shut down in flight, but which is protected by a fuse and circuit breakers so it shuts itself down if it draws excessive current or gets too hot? What is the risk from a malicious pilot? IMO the safest low-power electrical installations are probably much more reliable than human brains.
One thing to factor in is the much larger number of lives at risk on the ground from a suicide-pilot, compared to the fairly random crash location of a plane on fire that fails to reach an airport in time.
Oh, and instead of meaningless pings, have the antenna transmit the plane's GPS coordinates even when ACARS is shut down! How much extra bandwidth would mere GPS coords every half-hour actually cost? If this plane's black boxes are never found, this will be an accident or incident with unknown causes that may recur, so there is a strong interest in knowing where to look for a lost plane.