What the aviation study show is that the impair can continue for a very long time after a single small use. A blood test might be a terrible indicator how how much the impairment is, but the question then is how much tolerance you need in order to have zero impairment under some levels of THC. It also but a bit of doubt that even after 24hrs the impairment can be noticed, long past where the user subjectively feel any effect.

Alcohol tolerance might even be a positive here, since a drinker can drink a glass of beer 24hrs before a flight and be fairly certain that the ethanol has been broken down, regardless of tolerance. If THC metabolize slower as the body builds tolerance, then the impairment may continue for a longer time at a lower intensity even for a small dose, increasing the period of uncertainty.

The question then becomes "what level of impairment is tolerable".

Driving doesn't require perfect cognition, just good enough. If we went for perfect then anyone over 65 would be banned from operating a car.

And I think that's the hard thing with THC. Yes you may be impaired 24 hours later, but how impaired and how does that compare to age related impairment.