They'll always remain new cars until someone buys them.
When a new model year is about to come out, the previous model year often gets special incentives. That is generally time limited though.
A model that sits there and doesn't sell for a long time will lose those incentives. It was also be harder for the dealership to sell them then.
So, sometimes, they themselves will buy them out. Then they become "used" cars technically. And then they will "CPO" them, i.e. make them "certified pre-owned". Those usually get financing incentives.
But if they don't do that, the car will be still be new, but an older model year, and be MORE expensive to buy than a brand new model year car.