Air NZ rejected the voluntary severance of around 40 777 Captains because it's too expensive.
If redundancies were by fleet and not last on first off, they would be liable for Redundancy payments for a larger number of 777 Captains (and FO's) than they just rejected. It would of cost them an awful lot more to do the redundancies by fleet than by seniority.
The cost of down-training isn't a real cost while we're all sitting around at home doing nothing. It's more the logistical timeframe that makes it undesirable.