Fast food is designed to be addictive (salt/fat/sugar triangle), so the dichotomy is not really false. Its possible to be addicted to fast food. I point to obesity in America and draw some supporting evidence there.

I don't think "less hard on your digestive system" is an advantage but rather a disadvantage. Just like "less hard on you bones" is not really an advantage for astronauts that spend a couple of weeks/months in space.

If you want to have a healthy body you have to put it under a stress level for which it was designed by evolution. I.e. exercise once in a while and don't just sit in front of your computer, eat enough fibres and harder to digest food, etc

A lot of our digestive flora actually uses "non-digestible" components for sustenance, we're finding. There can be surprising and unexpected side effects to eliminating food components we thought we just excreted. Fiber, for instance, is technically not all digested -- much just passes through -- but it's really important! Other types of fiber are fermented in the gut depending on what bacteria you've got.

Our bodies having to break down food is not necessarily bad. For example, it allows you to receive a steady stream of nutrients throughout the day, instead of getting it all at once and being deprived later on.