I don't think that those objections are sufficient to draw a conclusion that quickly doubling the amount of waste product of our respiration in the air we are breathing in has no effect. Especially since people are not just briefly exposed to it but almost always suffer it from the first breath they take on this planet to the last. Without very strong evidence I wouldn't dare to assume that levels of this waste product that our species never experienced have no detrimental effect on our most vulnerable systems.
There are a ton of reasons not to want to increase atmospheric CO2, but I don’t think direct human health impact is something to worry about.
Since humans have been building shelter and living in caves, we have regularly been exposed to long term CO2 levels of 1000-2000ppm. The natural variations in ventilation dwarf a few hundred extra ppm in the atmosphere.
We’ve also been exposing submariners to thousands of ppm for extended periods with no observed effect, and we have many studies shows no observed effects up to 40k ppm.
We even have studies showing that small babies sleeping next to the mothers are exposed to CO2 levels of 5-10kppm.
