[schiffern]

>tried-and-tested technology

His objection isn't based on technology though. It's based on physically enveloping the problem and comparing the two cases.

His assumptions seem to be: LOX will remain cheap. The atmosphere will still have 21% oxygen. Your technology can't separate the N2 from the O2 before ingestion.

Elon says that the braking effect of ingesting that nitrogen vastly overwhelms the small benefit of having a smaller O2 tank. That's it.

This argument doesn't rely on the air-breathing stuff being more complicated or expensive or heavy (though it will be). Technology might fix those problems, but that simple kinematic fact is enough.