All sorts of factors would have been considered here.
The company, with a tip-off from the police will need to weigh up whether it is worth checking in and loading up a flight with passengers, cargo and luggage on the off-chance that the cabin crew may or may not fail a breath-test at -40 mins.
MAN is pretty much an outstation to the company, so there are no crew on hand to call out, so cancelling the flight must have been the best option to them at the time.
If you were a passenger on that flight, would you want BA to wait until the last minute to catch them out, or would you rather BA made use of the notice time and re-booked you, so you could get to JFK on time, or close to it?
What happens to the crew now will be determined after a lengthy investigation conducted by the company and overseen by the unions. Most likely, ALL of them will be suspended on basic pay. It will be determined whether any of them have a case to answer. Clearly innocent parties will be re-instated. Then there will be a dragged out "he said, she said" with possible terminations and/or demotions.