[kazinator]

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.

[telgareith]

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)