The passenger masks are not intended to sustain you for a long period of time. They are a supplement to help during the time that it takes for the airplane to descend to an altitude with a higher air pressure.

The problem is the partial pressure of oxygen. Only a small portion of the air is oxygen. To make it easy, say 20%. If the atmospheric pressure is 7 psi than the partial-pressure of the oxygen is 20% * 7 psi or 1.4 psi. If you reduce the air pressure to 3 psi by going to a higher altitude then the oxygen partial-pressure drops to 20% * 3 psi or 0.6 psi. At some point the oxygen partial-pressure drops to the point that the oxygen in the lungs stops moving into the blood and you blood oxygen level drops.

There are two ways to solve this problem. One is to increase the pressure of the air with the use of a pressurization system. The other is to increase the percentage of oxygen in the air. If you increase the oxygen to 50% at 3 psi then your oxygen partial pressure is 1.5 psi or right about where it was at a 7 psi atmosphere.

At some point, 100% oxygen is no longer enough to produce an adequate O2 PP so pressure-demand breathing systems must be used which provide 100% at an increased pressure.

In the case were the O2 PP is insufficient, you're still much better off breather 100% O2 than breather air as it will still increase your O2 PP by about 500%. It may not be enough to sustain you for a long period of time but your blood oxygenation will be much higher over the short term than it would be without the oxygen. The flight crew, of course, has pressure-demand Eugen masks which they would don immediately.

If the ambient pressure in Concord reached 60,000' everyone would be passed out or dead. The airframe and pressurization system was designed so that failures would not result in a complete loss of pressurization. IOW, the engines were capable of pumping in more air than could leak out in any foreseeable problem. That's why the windows were so small. Additionally, the airframe was designed so as to stop any failures from spreading and resulting in leaks in excess of the pressurization system's capability.