De-growth shouldn't significantly lower average life expectancy - after all in developed nations a high percentage of deaths are due to lifestyle decisions that are only possible because of our high use of technology and exogenous energy sources. As long as we still retain the knowledge on how to treat most diseases, and the physical infrastructure of our health-care systems, there's good reason to suppose average human health/life expectancy might well improve with the right sort and amount of de-growth. "Quality of life" is much harder to judge. It would definitely feel like a step down for many people used to just having everything and anything they want delivered to their door at the click of a mouse button. And it's true there's no good example that I know of a large population smoothly transitioning into a lifestyle with a significantly lower material standard of living. But I'm not convinced it couldn't be achieved in such a way that most people would come to accept that we've gained more than we've lost.