I would love to be noble and say of course I would move in and help my grandmother with dementia, but honestly? I would not. Not as a younger person and not as an older person. Taking care of an cognitively challenged person is a full-time job.
I should know. My 14yo son is mentally handicapped. He is not independent in his functioning like your grandma, but then again, your grandma will eventually not be fully functioning either. Her situation is not going to get any better, only more demanding for you all. I love my son dearly, but he needs 24/7 supervision. He has to be fed, bathed, played with, cared for, diapered. If your grandma lives long enough, this will be your life, too. I cannot leave the house unless my son is attended. I cannot take him to concerts, church, weddings or funerals because he doesn't understand and gets disruptive. I love him, but living with a dependent person has completely taken over my life and the lives of my children. Sometimes I throw myself a pity party for one, because my friends don't understand. Our life is soooo different from theirs. Most of them have not been able to hang in there with us for the long haul.
I commend you for considering this. If you do move it I would strongly encourage you to get some caregiver assistance. We just this year were able to get a CNA who comes in to help with Christian. Most of the time we don't even leave the house. But it is so helpful to me to have that 3 hours "off" to do something else, even watch TV or check email. My blessings upon you whatever you decide.