Commercial airline cabins are pressurized to at most 8k feet; there are no hypoxic effects at that altitude, at least not on decision-making. You don't need continuous oxygen on the flight deck until either 14 or 14.5k feet (in the US, EU might be slightly different). The FAA is dogmatic and wrong about a lot of things, but if there was even the slightest bit of evidence showing decreased decision-making or cognition at lower altitudes, they'd be requiring oxygen immediately, economic consequences be damned.