Earlier quoted context omitted.
O2 levels are not set that way. Just think where all the new CO2 is coming from.
Much of it is coming from the carbon in fossilized remains of a time when there was much more O2 in the atmosphere because there was so much flora and a warmer planet... what am I missing? https://en.wikipedia.org/wiki/Geological_history_of_oxygen
Or everything goes to hell in a handbasket and earth looks like mars in a few thousand years. Who knows.