> How the R word was a medical condition, how the N word was a normal thing to call somebody, how a gay man was called the F word. Nobody would bat an eye at these words back then. And yet - we look back on those times with disdain.
I think that you are conflating two different things, and somewhat mixing cause and effect. We do not look back at those times with disdain because they used those words. Rather, we look at those words with disdain because of the attitudes with which they were used in the past.
Furthermore, those three words fall into somewhat different categories. While the [acronym of ginger] and the [en_GB cigarette] words were overwhelming used from the outset to target and attack people with no moral justification (under our modern sensibilities), that is not the same as the term retard which was a legitimate medical term to describe an individual with mental retardation. The fact that we now reject the use of that word is precisely because it became a convenient pejorative to insult somebody's mental capacity, hence it being the perfect example of the euphemism treadmill—in fact, retard itself arose as a euphemism for slow.
Will the euphemism treadmill catch up on the other two examples? Will "gay" or "lesbian", or "black" or "African American" become taboo? I bet they won't, for the simple reason that as a society we have (mostly) moved past the idea that there's anything wrong with being sexually attracted to people of one's own gender, or descending from Sub-Saharan Africa.
Regarding people in the future, if they do look at us with disdain for using the words fat, ugly, mad, etc, my bet is that it will only be for the euphemism treadmill effect, as there is something inherently negative about being overweight, unattractive, or mentally unhealthy. I find it hard to believe that any future euphemism for any of these three words could ever become immune to becoming an insult.