Pfizer behaves very differently with an increase in marginal revenue than a patient (who is passed the cost from the distributor) would behave with a similar decrease in health care costs.

Anticipating pill expiration and what will be excess supply is a difficult predictive modeling problem. At a practical level, it's difficult for pill expiration date to make it into the price, especially when the purchasers of the pill are different people than those who throw out the pill years down the line.