In your question you stated the running out as a given fact ("When" we run out, not "if").
If that was what you wanted to say I can't tell you, but that's definitely how it was received and thus you also got the harsh response. Since it reads a lot like doomsday thinking.
(Example: Does that mean when we run out of oxygen there are no more humans?
Why would we run out?)
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.