Inspect all the gearboxes, assess which ones show signs of distress and note their running hours then adjust the TBO for the MRGB downwards across the fleet appropriately - replacing the epicyclic stages on condition.
Expensive? yes but not as bad as redesigning the whole box or scrapping the fleet.
That would be fine if the failures tend to occur on gearboxes with high hours. But do we know that is the case with this accident? ISTR that of the two bevel gear shaft failures, one was quite high hours but wasn't one quite low hours?
If a newish gearbox can fail big-time with low hours and without warning, whilst other gearboxes of the same design regularly run to TBO, then that is a bit worrying. But surely under that circumstance it would be likely a flaw at manufacture, in which case the remedy is to tighten up on the quality control at the factory.