This talk of the Captain not being there, and taking his rest, is a red herring, as far as I'm concerned. If the aircraft had crashed at some other point in the flight for some other reason, you would argue exactly the same point.
I regularly "punched" my way through the ITCZ, as an FO, on a 747, heading to Jo'burg etc. with the other FO sat next to me, and the Captain sound asleep, in the bunk. Occasionally, between the two of us, we even had the wit to deviate several hundred miles, past weather.
To be an FO on long haul in my airline, you need to have an ATPL, be qualified to P1 standard, and have several thousand hours experience. For sure, the Captain is in charge, but he has to delegate that responsibility, make sure his FO's are briefed, and are happy to call him if they feel the need. Using the weather radar, and course deviations are not really a big deal. They happen all over the place, as can any other emergency, whilst the Captain is asleep.
I am now a Captain on short haul, and to judge by some of the comments on this thread, I'd better never go to the toilet, in case something happens, whilst I'm chatting to the CC, and the "200 hour wonder", or whatever you want to call them is at the controls.
These AF FOs were deemed qualified to act as P1, by their company, whilst the Captain took his rest. They could easily have been captains, on another fleet, or in another airline. Let's move away from this "man and boy" concept, and "if only the captain was there" rubbish. Frankly, if you's seen some of the captains' flying skills I've seen, you wouldn't be saying such things!
Of course there is the emotional concept of "being responsible" when one is a captain, and I take that very seriously, but I doubt anyone wants to die really, do they? When I was an FO, I would frequently look left, for advice, whilst now, I only see my own reflection, but hopefully always remember to then look right, and share the problem. These sorts of moments are almost invariably not QRH actions, or "flying the aircraft" ones, but more typically, tricky ones on the ground, involving ATC, passengers etc.
There should be no reason why 2 experienced FOs could not navigate, using the weather radar, or fly their aeroplane equally as well, if not better than their captain.