The improvement at the upper end isn't even that much in absolute terms. I guess it's hard to imagine accurately, but I think that when I'm 85, I'm going to be a lot more worried about the quality of my next few years, rather than whether there are going to be 4 or 6 of them. Four high-quality years would be far preferable to six questionable ones. (Of course, six high-quality ones would be even better, but if I had to choose, I'd choose quality.)