Thank you for sharing the story of your folks. 71 years is a long time.
Yeah, dementia is tough. Yet...there is nothing I can see or think of that, if it were changed, would make his life, such as it is, any better. And it could easily make it worse.
His doctor says: If he is eating and has a smile on his face now and then, he’s doing fine. He has a ravenous appetite (eating being probably the only thing he has of any “feel” nature) which is good. I will tell you that I have thought he was at a “knee in the curve” in his overall health 47 times and was about to enter a steep decline towards the inevitable. And every time, he just slows down a teeny bit and gets little bit less chatty...but that’s about it.