Let's just fix it and then, if really there was some consensus to be reached, we will revert (it won't happen).

Why not add an option in the media viewer itself? In that way every user can opt-in (or opt-out) and decide how they want the media viewer to work. Also, you could gauge how many people prefer the "nuke history" type of behavior and how many prefer the current behavior instead.

Kind of. However note that the information published in T64266 seems to be outdated: it's said that Facebook nukes the history entries when exiting the lightbox, however this doesn't seem to be the case anymore.

Or, you could just simulate the back button when hitting the "X" button (or the Esc key). I think this is what Facebook actually does. I think it would also be easier to implement.