I don't think your second point is up for argument. We're not trying to disambiguate whether doctors are getting better at treating COVID or COVID is itself get

I don't think your second point is up for argument. We're not trying to disambiguate whether doctors are getting better at treating COVID or COVID is itself getting easier for doctors to treat. Age is a strong predictor of COVID fatality, either directly or through correlates. In every US county whose stats I've spot-checked, deaths go up with age. It's also one of the best predictors, given that advanced age is much more common in populations than stronger, but more selective factors. Thus, controlling for age and nevertheless observing a reduction in mortality should be taken as an indicator that COVID is becoming comparatively more medically tractable.