The CO2 content of the athmosphere is actually not that high, nor will it be at the end of the century even if we don't reduce global emissions. The problem is that it is quite enough already to cause climate change. Which would also not be such a big deal if not coupled with large-scale destruction of ecosystems and our dependence on non-renewable resources.
That's an interesting way to phrase it. If the atmosphere were going to saturate in a way that would prevent combustion from even happening that would be a very interesting scenario.
if you are thinking along the lines of atmosphere saturation to a point of not being able to support combustion, then i guess "living" organisms who depend on oxygen would be long gone by then