My father lived with us for about 6 years until he requested going into an assisted living facility. He said he thought we deserved to have more privacy and less obligations...............so we could travel and go out from time to time. He was 90 then. Now he is 92 and lives in an adult family home, because he has advanced dementia and is blind. The timing of it all (the moves) turned out really well.
Mom lived with us for about two years of the six that Dad was here (they are divorced but friends). She needed more care because of osteoporosis, so also went to assisted living. She couldn't maneuver the stairway in our house any longer, at that time. Dad went there about 3 months after Mom did. Mom had surgery about a year later and lost her spot at the assisted living place and is also now in an adult family home.
It actually worked out very well having them here. Of course we had no privacy to speak of, and it meant more work for me in meal preparation and laundry, cleaning, etc., but they were pretty respectful of our space. Dad had to be reminded at times, that he couldn't just "pop in" on us at any old time. They lived in our downstairs which gave them two bedrooms, a private bathroom, and a very large living room. There was a kitchen too, but Mom only used it to make her breakfast and did some baking. Dad ate with us unless Mom felt like fixing something for him too.
It was the right thing to do for them, and I will never regret it. Sometimes they have no other options. If he lives another three years, so be it. I doubt you will have any regrets after he is gone. My concern in this situation is he wouldn't be able to see his wife. Can he go into an adult family home in his town?
I almost forgot that my mother in law lived with us for several years too, before she remarried. Don't laugh, but she married my father. They were both 83 at the time. She passed away about 6 years later. She was a dream to have here. It was wonderful for our youngest son, who was 9 when she moved in. He spent every evening with her, watching TV and eating special snacks. It allowed them to form a very strong bond. I always said that my mother in law was the perfect MIL to have. She was just great. She worked full time until she was about 81, and was very high energy. She was a cake decorator and loved it so much she couldn't bear to retire.