"Western society" as in the US and maybe the UK?
Because in most other countries people do not rely on mass produced food to feed themselves.
The public health debacle is worst in the US because the US federal government massively subsidizes production of sugar as a way to use up what of its (also massively subsidized) maize production is not forced on every kind of livestock, or exported.
Ironically, regular corn sugar, dextrose, is relatively harmless. It becomes poison only via treatment with the enzyme that turns half of it to fructose.
If you ignore common knowledge about food, then just as with ignoring advice about exercise, maintaining your teeth, and many other things, you will probably suffer. Good health was never presumed to be the baseline regardless of what one ate, it always took persistence and impulse control. The question is why people are doing such a poor job at it now.
In particular, they had no difficulty getting enough fiber: there was plenty in everything. Our problem is that our whole food-industrial complex is devoted to stripping out and discarding the fiber we need.
If 40% of Americans didn't brush their teeth or bathe, we would have a hygiene problem of a magnitude commensurate with our health problem.
The value of fiber (apart from keeping some nutrients indigistible and hence less calories for obese people who need to eat less) is still a matter of debate.