Scrubbing the CO2 from the air has no effect on it. The CO2 that tells you to breathe is not in the air. You exhale the CO2 because you specifically do not want it, you need to get rid of it. Hence the scrubbing.
If there is not enough oxygen in the air to breathe, having more CO2 in the air will not save you.
If we mix, say, 25% CO2 and 75% regular air, we will get something which still has oxygen, but at too low a concentration to support breathing: the oxygen will have dropped from 21% to around 16%. That's still a lot of oxygen, but not at a breathable concentration. If all the CO2 is removed, then the breathable air is recovered.
Simple rebreathers use pure oxygen, and even when they don't- as long as co2 is steadily scrubbed out, its little more than a regular scuba regulator.
Oxygen has to go above a certain partial pressure (happens at a couple meters down, I don't know the numbers of the top of my head) before it's toxic. And, carrying a tank that's got more than oxygen in it defeats the original military development and use model of rebreathers: no bubbles, and long submersion times.
The real problem for users is a lot of carbon dioxide scrubbing compounds will kill you if they get wet.
Engineering wise, I seem to recall pressure balancing (countering the water's pressure), and forcing your exhalation air through the scrubber being the complexity.
Theres a ton of complexity with rebreathers. And, thats before accounting for the fact they're mostly used in combat and cave diving. (Last I checked. Tbf, I got scared and changed career paths away from the sea after Rouge Waves became irrefutable fact)
You don’t need co2 in the air to tell you to breathe. Your body makes co2, which is what you feel. Once you run out of oxygen to make co2, you are already unconscious.
If you go into an area with no oxygen, you will pass out very quickly with little warning except being dizzy, but you will not fail to feel like you need to breathe.
It might be slightly more safe to enter into a room full of co2 than a room full of nitrogen, because you might be alerted by the high co2 a few seconds before the dizziness takes over, but that doesn’t make rebreathers dangerous because you will forget to breathe. If you run out of oxygen, you’re unconscious in a few seconds anyway, co2 or no co2.