That's really the primary driver behind cancel culture in general. Without agreeing with the concept, the point stands that celebrities really can't be afforded the benefit of the doubt when it comes to apologies, even unprompted ones, because they have too much capital (real and emotional) invested in an artificial, public persona. The apology has to come from the person not his or her avatar, and, for better or worse, cancelling an individual at least in part strips away that artificial front. Unfortunately, the larger issue is that cancel culture doesn't allow for a space for honest apologies once the artificial image is destroyed.
Personally, I don't hold Harmon to that kind of standard because he's a particularly dark comedian. For better or worse, he's made a living off of making some really cynical, if not outright nihilistic, jokes, so it's to be expected that he's eventually going to make a joke that goes too far and he's not going to apologize for it simply because its vile in it's own right. It's only somewhat comparable, but this isn't unlike when Gilbert Gottfried made a joke about Japanese tsunami victims and was forced out of his iconic pitchman job with AFLAC. Anyone who's seen Gottfried over the years expected him to make that joke, but that doesn't mean it was any more acceptable once he did.