With the oxygen toxicity at too high pressures I agree. But you've got some weird understanding of the breathing reflex. Yes, breathing is based on CO2, in the sense that high concentrations of it cause you to breathe. The way you describe it, it looks as if it were the other way around, that is low concentrations of CO2 caused you to breathe(or high concentrations caused you to cease breathing), which doesn't make sense.
Sorry about the misconception, I only stated that breathing was based on CO2 concentration. The reason you would keep breathing is that while you have too much 02 coming in, you would be expiring roughly the same amount of CO2 (the trace amount of CO2 in our atmosphere has negligible effect on the concentration gradient). Therefore, even though you have too much oxygen, you keep breathing because your body is still generating CO2 That needs to be expelled.
I think it would be rather nice if we were able to visit, or perhaps even terraform and colonize, Mars. I don't know much about the minerals there except that the surface is covered in oxidized iron, but I'm sure we could probably find some reason to make it worthwhile, even if it's just to be able to wave our giant (inter)national
penis spaceship around in everyone else's face.