Bingo. Quantity is worthless without sufficient quality. I would rather be healthy and strong and die at 80 than sickly and bedridden for the remaining decades and die at 120.
(And for my part, I'll always take those extra 40 years, no matter the condition, because that's 40 more years for medical science to advance and solve more of those problems.)
Will it? I don't see why it necessarily will do so.
And even if it does... if it gives you 10 more healthy years, and then 20 more bedridden years, for a net gain of 30 years, is that really a gain? Or do the 20 bedridden years make the 10 healthy years not worth it?
And yes, of course it's a gain. You could always choose not to take those extra years, if you really don't want them. It's good to have that possibility available.