It might be that poor ventilation gives me headaches for other reasons, and CO2 concentration is just a useful proxy that's easy to measure. The evidence seems to support this idea, whereas like you said the evidence for CO2 in isolation is pretty questionable.

If nothing else, at least a well-ventilated room expels my farts and other airborne waste products.

And people don't want to sleep longer, they want to feel well-rested. So I think higher sleep efficiency is better.