> I am unsure whether a jet liner can descend to a breathable altitude (say, 10000 feet) and feed "fresh", or ambient, air into the plane. That seems like a nice redundancy to have and I wonder how realistic it would be to implement ...

This is already in place, and has been since planes began going high enough to require pressurized cabins. The engines basically suck in lots of air which is compressed, resulting in a higher temperature. Most of this goes into the combustion, but some of it goes into other applications, such as the environmental systems supplying the cabin with air.

It doesn't have the same effect as opening all the windows in a car, but considering the outside temperature[1] that might not be such a bad thing.

[1] - https://www.engineeringtoolbox.com/standard-atmosphere-d_604...