IIRC CO2 is toxic because it raises the blood pH (carbonic acid). SO in a forced low concentration (20% or so) CO2 might be worse. OTOH, a room full of N2 would be worse than a room full of CO2. As you point out, the body regulates oxygen intake by monitoring the CO2 in the blood (pH). If N2 replaced the O2 the body wouldn't react by demanding more oxygen, causing the brain to be starved of oxygen. The subject would go into hypoxia and not even know it (Payne Stewart). OTOH, if the CO2 concentration got too high one naturally seeks more oxygen (lower blood pH). In this case panic is better than bliss.