Considering humans as exemplary of oxygen breathing life, the issue for us is the gradient of CO2. We generate CO2 and to expel it, the concentration in exhaled air must be greater than the concentration in ambient air. We cannot expel CO2 against a gradient. This author estimates the toxic level of atmospheric CO2 to be 6%.
Now we need to make connection between the partial pressure of CO2 in
the air and in the blood, before breathing in and after breathing out.
For the gas molecule to cross from the lungs to the blood it needs to
have the higher partial pressure in the lungs than in the blood, and
obviously, the opposite is true – if the partial pressure of the gas
molecule is higher in the blood than in the lungs, it will cross from
the blood to the lungs. When it comes to CO2, its concentration in the
blood, after the blood has collected all the CO2 generated by the
bio-chemical process keeping the cells alive reaches the pressure of
45 mm Hg, while the pressure inside the lungs after we breathe in the
air is 0.3 mm Hg. Therefore, as long as the partial pressure of CO2 in
the air that we breathe in is less than 45 mmHg, the human body will
be able to clear out the cell-produced CO2. By the way, when people
suffer serious brain damage which affects breathing, the function of
those machines that maintain the life is to bring in the oxygen and
make sure that all the CO2 is cleared from the blood stream. The
number to watch for is 45 mm Hg of CO2 in the air, or 6% or 60,000 PPM
– that is the concentration of CO2 that needs to be reached for the
humankind to become extinct. If my math is serving me right, if we
divide 60,000 PPM with 400 PPM we get the ‘kill factor’ for CO2: 150.
Humans tolerate elevated levels of CO2 OK. People with sleep apnea or emphysema compensate for elevated levels of CO2 via various metabolic means.
This piece of the answer is meant to address the oxygen breathing life component of your question - the answer: we will do ok.
As regards why the planet might have a stably high level of CO2 that turns on issues of input and output. Where does the CO2 come from and where does it go. It is the same for any budget: income vs expenses. The atmosphere used to have loads of CO2. Then photosynthetic organisms chipped away at it for a billion years or so. If you want more CO2 you could
Increase CO2 addition to the atmosphere - from combustion, from increased degradation of carbonate rocks, from volcanic outgassing, from extraterrestrial inputs.
Decrease CO2 sequestration. The main consumer of CO2 is photosynthetic organisms. You could cripple photosynthetic CO2 fixers somehow; perhaps low light? Nutrient scarcity? Increased ambient radiation, UV or ionizing? Ocean got too hot or too acidic? Any methods such that additions exceed more than the photosynthesizers can fix.
Another consumer of CO2 is water - it is soluble. The oceans act as a buffer. You can do an entire PhD on this subject and so will not provide links. The ability of water to solvate CO2 (or any gas) increases as water temperature decreases. Hot water dissolves less gas. If the oceans warmed up they would hold less CO2. A warmer and more acidic ocean could theoretically cripple oceanic photosynthesizers in the short to intermediate term which might help with that aspect of your world.
You could have increased volcanic activity, increased outgassing of internal CO2, increased breakdown of carbonate rocks and also heat up the oceans nicely from below if you had the earths core start to heat up. I was wondering about this in the context of a previous question and so asked on the physics stack:
The answer is a little heady for me but I take away yes - if there is a decay product with a shorter half-life, it is possible for total decay heat to temporarily increase. Your warming earth would warm from the inside out and could lead to the above described changes affecting CO2 homeostasis.