O Son, You will die only at your wish. No one, not even Yama, the god of Death can claim your life, unless you wish to hand it over to him. How can Shantanu, who is a mere Human deliver such a boon like iccha mrityu, which is like onto the boon to be immortal, which even Brahma and Vishnu fails to provide?
It is true that Yogmaya took birth in human form, gave pain to her mother for nine months, and thus she will not turn out the rule of the world and go away by just alarming Kamsa, and that has been earlier established by us in other article. Since she has taken birth, she will complete the human cycle and then only leave.