Accept it man. They weren't going to pay Nicholson so much money again.
Now the same happened with Two-Face and he didn't even have as much screentime as Jack's Joker. He ended up dead by Batman's actions after their first confrontation. But what a great story it was.
In many good stories villiains are killed at the end. There's no rule about it. But as I said, in comics they simply acn't do that. B89 was thought by Burton as a self-contained story, not a first part of something bigger. It was much later than Burton even considered to make a second one.