Right, so I don't think we would completely die out. As always with almost any catastrophe, there would always be small pockets of humans surviving somewhere and I think it's reasonable to say they would rebuild eventually.

I'm just thinking that if the oxygen levels drop a few percent that won't kill us, but it will start affecting our cognitive levels - so I'm imagining that those who can afford it will just carry portable oxygen bottles with them. Offices/homes will be equipped with oxygen disperses through the ventilation systems, just so we can feel "normal". I imagine that would cause massive unrest with people who cannot afford this.

But hey, this is just speculation. Maybe we will find a technological solution long before this happens.