I think you have to have a resident visa, my mom updated hers several years ago.
But the downside? She cannot vote or that is what she has told me, and as much as she is into politics I think that this would be an incentive for her to apply for her citizenship, but in her words, "I have my reasons little girl"
Maybe sentimentality. That can be pretty strong. My family members are all "sentimental fools". Me included I guess.