I have seen an actual photo of the sky-morgue on the A345, which Singapore used. Obviously, a passenger dying in-flight can and has happened, and it's no doubt a better option than spending hours of flight-time and ( hate to point it out ) burning thousands of tons of fuel to divert somewhere, particularly on very long-haul oceanic routes.
Besides, if a crew member INSISTS that a passenger needs to cool it and "chill out", then they can have some bite in the threat ! "Chill out, Mr. Passenger, or we have got JUST the perfect seating place for you...!"
And then, of course... "NO ! You cannot, repeat, CAN
'T smoke in the sky-morgue... so don't ask us again !"