I've never understand that attidue, my grandad died when he was 79, when he was 78 and quite ill he learn't how to use the internet, was driving all over the country with my grandma and kept very busy. He enjoyed his final months although he didn't know he was dying as he thought the operation would cure him rather than kill him.
My dads parents are like your nan though, ever since my mum met them in the late 1970's they have been moaning they are on deaths door, in 2006 they are still alive (just) and both well into their mid 80's.
One thing I will say is that I saw a lot of my grandma and grandad and I am really glad I did now as every memory I have of my grandad was precious. Although I was very close to my grandad at least I had no guilt.
With my dads parents which I rarely see I reckon I will see quite guilty when they go.