Certainly I won't argue with the very-old being a drain on society reasoning. To some extent it can certainly be true -- e.g. workforce, taxes, economy. Whether or not that is countered in terms of wisdom, historical knowledge, and otherwise unobtainable perspective is a subjective matter.
I do, however, take umbrage with the idea that remembering someone as "vibrant and engaged" is a good thing. Everyone that I know who's died "vibrant and engaged" has been the result of some crime or major illness, and has left friends and family distrought to the point of needing some amount of psychological therapy to get over the loss, sudden or otherwise.
The idea of a very gradual decline, such that finally losing one's grandfather comes when one's opinion of that grandfather is at least somewhat "feeble, ineffectual, even pathetic" is a comfort. It makes the loss easier, understandable, and acceptable.
Moreover, I'm 35 now. I'm not feeble, but I'm not fit. I'm not inefectual, but I'm lazy. I'm not pathetic, but, well, to some I am. I'm a pretty relaxed, happy guy, with no problems and no ambition and a lot of personal hobbies. If I cared to be seen as "vibrant and engaged", I wouldn't be content as I am today. That would be horrible. I don't live for the memory of others; I live for my own joy of the day.