If cost(t) >> cost(t+1), then I'm not going to be able to afford to buy that new car. I'm going to be underwater on that deal. That also means that Tesla has one fewer possible sale. I'm not overpaying for a new car, which means that if Tesla would like to have more sales, they should try to keep their prices more constant.

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).