If Tesla is dropping the price of their cars, then that means you can buy a new car for less. It makes no sense to want to overpay for a consumable item at time t+1, regardless of what you paid at time t.
Remember, this is from the Tesla point of view... and it's a long time-horizon perspective. It's a much easier decision from the customer's point of view.
(And there are other ways for companies to incentivize sales with discounts and promotions that don't obviously impact the "value" of the car).