My m-i-l had dementia and ate terrible and smoked like a chimney in her latter years. She was about 85 when she passed.
We tried to feed her well, but what a fight it was.
The doctor, being very realistic, just said to do the best we could. At that point, there was not much we were going to be able to do to change her, and considering her age, there just wasn’t any point in trying too hard to improve her diet or getting her to quit smoking. The damage was done and we might as well just let her enjoy her few years left. They had enough experience with dementia patients to know that.
It’s hard to watch, but it is what it is.
Sorry to hear about your MIL. You are right. Sometimes things just are what they are.