Because all this involves sensitive incidents and decisions, we're keeping unspecific.
Its just not financially or operationally feasible to scrap fully functioning cars because of their involvement in tragedies. We'd have no cars left it we removed it from service for every fatality. I believe the decisions made to retain the cars were most prudent.
As for this Westbury incident, as much a fan of the M3s I am, I am happy that this involved M7 equipment, for the safety of the Engineer and passenger in the lead car. The M3s do not have the collision posts and crash-worthiness that the M7 does. Bizarrely, the platform cement penetrated the car between the two cab-side collision posts. Had it been an M3, the whole carbody would have been cut open like a soup can with a can opener. In my opinion, larger amounts of concrete would have entered the car, with greater ease.