You still seem to be missing the point.
If you talk about "when we run out", you are presenting yourself as an expert stating "we will run out" and asking about the aftermath.
It would be appropriate, and better received with more leniency, for you to ask whether we would run out.
Of oxygen, because of rising temperatures interacting with rock weathering binding all the oxygen.
Now, that's more of something to worry about at geological time scales, but Earth in fact, is not infinite.