I don't know how much of the CO2 is taken up by plants nor do I know what percentage of CO2 produced by rot is used.
There are other mechanisms at work. I read somewhere that the ocean takes up CO2 and turns it into CaCO3.
I am a biologist, and have specialized in invertebrates. So I am no expert on this stuff. I just wanted to confirm your thought that there is a cycle at work that the wackos ignore.
Also, too high an O2 content in the atmosphere would kill most live on Earth.
life not live