Yes, dementia is a tough thing to deal with, and I am sorry that your father suffered from it. Are you thinking that BS had dementia, and was therefore, more liable to "snap" over the house move?
I don't know Barry so I can't say. But I can tell you that with my dad it was a very gradual slide, as I'd noticed things decades ago. But there were signs, like not wanting to throw things out. He ad a pair of sandals that were falling apart, but he just kept wrapping duct tape around them. There was old patio furniture (they had new stuff) that was piled a the back of the property. Nothing but rusted frames, yet he would have a conniption if I tried to lug them to the curb. The house was badly in need of a new roof, but he'd have none of it. When water started to pour through the kitchen light, he'd fix it by putting a wash tub in the attic to collect the water. I remind you that this was a multi-millionaire. And, he was sane enough to pass his driving exam during this period.
So was Barry's refusal to get a new car similar, or did he just prefer older things? Who knows?