The karmic retribution seal wouldn't kill the person affected by the seal, only remove their cultivation. Bai Yun would be turned into a mortal, and he could spend the rest of his life peacefully.
But Bai Yun was not going to do that. He might have been fine with that life in the past. But after the talk with the God of Death, he knew that it would be a mistake.
If he died a natural death as a mortal, then he would enter the reincarnation cycle and be reincarnated once again. But that was the problem. If he died as a mortal, then in his next life he would definitely be sensed by the heavenly dao when he is born. At that time the heavenly dao would once again place its shackles on him. He wouldn't be as talented as he was in his past life as Long Tian, but it would be second only to that. And as someone shackled by the heavenly dao, he would be once again its slave and be forced to defend against its threats until he dies.