If they knew it was coming I think a lot of people wouldn't have the motivation to stay in shape. I mean what's the point.
I think it would be the opposite, because it would be possible to be in decent shape pretty easily and still have a high degree of wellbeing. Maybe some would let themselves go more, but perhaps the incentive would shift away from life-prolonging medicine, to life-improving.
why wouldn't you spend the last two or three years of your life doing or not doing whatever you want?
For sure! Retire at 65 and then enjoy life, knowing that there is only 10 years left. you know how much you need, how much you can spend, and what support you will require from others. Live it up, rather than save it up, just in case.
I think you have more faith in humanity than I do :)