The potential benefit of any dental procedure necessitating general anesthesia doesn't outweigh the potential negative affect of the general anesthesia on someone with dementia, in my opinion.

My father has very few teeth left. By the time we got a diagnosis and I started being responsible for his care, his lack of dental hygiene had led to a mouth full of infected roots from which the teeth had broken off. We had those extracted using local anesthesia and he does just fine with his few remaining teeth and his gums. He eats a lot of soup - and puts leftovers into his soup bowl to soften them up so he can masticate them well enough.