This existing system wouldn't likely cause loss of all engines unless the PIC commanded all engines TR in-flight. Also, it doesn't make sense that such a system would go out untested because such a system should try command engines to idle before cutting fuel to what it thinks are runaway engines. Finally, it's still unknown if it's a Boeing software issue or a RR T1K issue... the OP article is purely speculative and guesswork "news" is not how aircraft safety is handled.

A single engine on reverse is enough to mess up the the flight violently enough that it'll break apart before hitting the ground (see the Lauda Air case).

I think there are a few options for a failsafe here.

none where it would deploy the reverse thrusters and probably crash regardless of other inputs to the system(could be one or more of many inputs wheel speed, slats deployed, airspeed, elevation, throttle position, etc.)

turning off the engine (presumably you want this because you are on the ground but your ground sensor is failing so you want to cut engine and apply brakes which is less preferable than the reverse thrusters but manageable normally.)

ignoring the input altogether.

not a failsafe at all and an unexpected failure mode of the system (I think this is probably the case since they couldn't relight the engine on the ground)

the point i was making is that if you have the failsafe turn off the engines under normal operating procedures it should be able to relight when in flight and it's not good if a software glitch turns your heavy into a glider without possibility to relight. (i am not a pilot but it's my understanding that you still have the turbines spinning and all you need to do is give it some fuel and fire up both ignition plugs. might need to use the compressor to spin them up to full speed but i doubt it.)

Have a mechanical latches on reverse cowls that will be operated separately of the code that deploys reverse. Landing without reverse is better than falling like a brick.