Amylynnet, as I read your questions, it appears you and your mother-in-law assume she will die when she stops her NPlate treatment. That isn't at all a guarantee. At her age, it's somewhat more likely than it would be if she were in her 40s, but it's still not a given.
Low platelets could make any bleeding that starts in the brain, stomach, or intestinal tract to become unstoppable; but that could happen even in the absence of ITP. The odds of having such a bleed are higher with old age, because the vessels are in a weaker state then than at younger ages; but low platelets alone don't usually precipitate a bleed. Does that make sense?
I'm 72. My ITP has never caused me any symptoms other than unprovoked bruising, petechiae, mild nose bleed, and mouth blood blisters at the very lowest counts. There have been times for me that low platelets didn't show any signs at all. I've never had a massive bleed of any kind, and never been hospitalized for the disease. When my count is above 20, I wouldn't be aware that I have ITP. For most people, there are signs that the count has become low. But unless massive bleeding begins, death would not be imminent or predictable.
I wish for her sake that someone could talk your mother-in-law into entering a hospice or palliative care program. My aunt is 93, in a clear and razor-sharp mental state. As of 10 weeks ago, she was still living independently and driving a car. Then she broke her hip and her heart condition has deteriorated to an insurmountable degree. She just made the hospice decision last week. It's not pleasant for her to know she's so close to the end of her days, but she is relieved that any suffering she's about to face will be supported by caring professionals looking out for her best interests. And she's most relieved that she won't have to shuttle back and forth to the hospital for futile treatments that won't improve her condition.
Wishing your MIL and all her family the best...