The "negotiation at the dealer" doesn't change. You get the rebate after you buy the car, the dealer doesn't get it. The dealer cost, and therefore profit, doesn't change when the rebate goes away, so whatever your best price you can negotiate today will be the same if the rebate goes away ...
I disagree. When you are negotiating with the dealer, the dealer knows (today) that you will get the tax credit, so will hold out for a higher price. When the tax credit goes away (which it will eventually, now or later) the dealer knows that you are not getting the tax credit and so cannot use that as a negotiating point.
The tax credit lowers your total cost, but works against you in the negotiation. When it ends, the absence of the credit becomes a point in your favor when negotiating. The dealer does not have a set price. He wants to get as much out of you as he can, and will hold out for more if he knows you're getting the tax credit.
In addition, if the end of the tax credit does result in reduced demand and there are cars sitting on the lot, the dealer will settle for a lower price to get them off the lot. Prices will drop when the tax credit ends, though they will not drop as much as the credit.
Tesla is a different story because there is no negotiating. I like that about Tesla.