Agreed - I think all those systems should get looked at. I don't doubt that - as with pretty much any major accident - numerous things would've had to go very wrong for it to happen the way it did. Heads should roll at Lufthansa Technik if they messed up or overlooked the oxygen system during the previous check.
The other incident to bring in here as a possible precedent would be
Helios 522. It would be shocking if it was a non-sudden loss of consciousness, considering lessons should've been learned from that one, but that's also plausible.