change in history had turned allies into enemies, with Worf trying to kill people who were his friends in the "right" universe.
Isn't that basically what Regent Worf was doing in the DS9 episodes? Garak may not have been Worf's friend in the RU, but a lot of those other people are.
The point is that it would've served the drama of that particular episode by providing another reason why it was so important to restore the "correct" timeline. That's not an issue in the Mirror Universe stories. Worf's Regency wasn't about exploring the tragedy of the altered circumstances, but about "Wouldn't it be fun to see Worf as a scenery-chewing bad guy?" And about "We're paying Michael Dorn a lot of money to show up every week, so we need to stick him into the MU somewhere."