After all, the purpose of moral disapproval of atrocities is simply to avoid offending anyone who could be personally connected to them.

Personal connection is in the mind, as you say later. I've been looking at the "It would have been me" aspect of the past, and I think it's mostly trained in.

A major reason that the Holocaust is taken very seriously is that there are people who believe that doing so will make a repetition less likely. I don't know how long it would take for that to fade out.

I also don't know how close we are to longevity tech, but when such exists, the past is presumably going to fade more slowly.

On the relativity of what is considered serious-- I think there's been a bit of a shift lately, but when you think about Hitler's atrocities, you probably mostly think about the Holocaust. He was also responsible for tens of millions of deaths as the result of WWII, but that doesn't get the same publicity, probably because building an empire is viewed as sort of normal behavior. Alexander the Great, Genghis Khan, Shaka Zulu, and Napoleon aren't usually counted as mass murderers.