Two things.Dragline wrote: I think you may be missing the point. There is always uncertainty in health in that (a) we cannot anticipate every negative medical outcome that may befall us and (b) we may be doing things that we now think are healthy for us that actually might not be or might just be neutral/inneffectual. This means that at a certain point of "living the healthiest life we can conceive" only may get us, say a 2% improvement whereas the uncertainty factor may be 5-10% (the percentages are for illustration only). The idea is that the level of uncertainty exceeds the potential benefit such that the effort becomes useless other than any psychological benefits.
1) We do not control everything. Absolutely. We agree on that. We know that kale is good and sugar is bad. Your 20% is a purely arbitrary number. A very high arbitrary number. That's 73 cheat days a year. It is 1/6 of the food you ingest as "bad habit" food. Alcohol, mind you, has the same metabolic cascade as fructose. So telling people it's okay to have a few drinks a day (20%) is like saying it is okay to eat spoonfuls of fructose. It is not.
2) Again, the 2% and 5-10% above is out of thin air. That said, there is certainly a point of diminishing returns to healthy eating and exercise. But just because that point of diminishing returns exists does not mean it is wise to discontinue healthfulness at that point. You have to work harder to get additional gains. Those additional gains may mean the difference between, for instance, my 75 year old friend who cycled Alp d'huez on a long cycle tour of Europe recently vs. my previous 75 year old tenant who can no longer walk because of weight. 2% a year for 20 years is a big variance.