Of all the dogs I have had in over 7 decades of my life, I have only had one with dementia. One was sadly stolen and never found; one young one was run over and killed; 3 were lost to cancer; one had dementia. The rest lived long and happy lives.
It was my borderline collie (99% border collie/1%spaniel) who developed dementia and it was upsetting to see. I'd find him standing still with no idea where he was, what direction to go in, straddling a low brick wall, snapping when caressed. He was in his teens by then, so it was part of the natural ageing process for him. In the end he became so confused it was kinder to comfort him as he was pts.
This is a weblink about recognising and caring for a dog with dementia.
A weblink to a book by someone living with a dog with dementia.