That's how it went for my mother in-law. My father in-law had dementia and was still physically fit. She had to keep a constant eye on him. He'd try and get in the car and go places. All the knobs and buttons were pushed. She had to start locking it and hiding the keys. He'd go outside and wander around the yard. He'd get up in the middle of the night and start dancing around the house and singing. He'd sleep all day. Finally around age 92 he stopped moving around so much and then he could no longer follow simple instructions/requests like "please come with me to the kitchen, bathroom" etc. He couldn't remember what a fork was for, so she had to feed him. Couldn't remember to use the bathroom, or where it was. It finally got too exhausting for her physically and mentally, and she was making plans to move him to hospice. We were waiting for a COVID test result (he had to test negative to get into Hospice at the time) when his heart finally gave out. He passed away with all of us by his side. 93. That was 2.5 years ago. You could see the relief on mom within a week. She still misses him, but the strain was too much for her.
We had a family member in her 90's who would escape the house and wander around. On at least one occasion she was found sitting in the middle of the street. It was early in the morning, in a residential neighborhood, so there was little traffic but even then, she could easily have been run over by a car.
Another person we know, and have tried to "support" for decades, has taken care of a family member with dementia. It is killing them. They have money to help get some help with care giving, and even put the person with dementia in a home, but they refuse to do so. The situation has had such a negative impact on them physically, but worse, mentally, that it is really taking years out of their life. I suspect the care giver is also scared witless that they will get dementia and have nobody to take care of them. It is a horrible situation.
Before we built our house, I would work on our land every weekend, and one of the neighbors would drive his tractor to visit with me every few months when he could leave his wife for a short time. He was taking care of his wife and I could see the negative impact on him when he would drop by. It really was obvious. He was going down hill pretty fast taking care of his wife and I was really worried about him. Thankfully, she died before he died or was unable to care of her anymore. He rebounded quickly. He was like a flower that was lacking water and then got some rain. It was amazing how fast he rebounded but I do think he was close to dying taking care of her.
After a year or so he started dating again. The odds were in his favor since there are more men than woman at his age. He eventually married a woman who had been a caregiver for years to her husband so they knew what each person had been through. Both of them had ridden or wanted to ride motorcycles but there deceased spouses did not want too or prevented them from riding. So they went out and took some motorcycle classes, got matching riding leathers and motorcycles after they got married.
Then he had to have a back operation and it all went down hill for him. He had several infections that almost killed him. He was in and out of the hospital and rehab for years. Eventually, his wife moved the dining room table, replaced it with a hospital bed and brought him home to take care of him. She took care of him for years while he lived in the dining room which had a big window looking outside so he had a view. I would wave at him when I would drive by. They had kept her house, so they sold his house, and moved to her house and we lost touch with them.
His wife was a saint to take care of two sick husbands, one after the other....