It's a little weird you say there is "genuinely no precedent" for possession, and then provide at least one concrete example. The title of the video is "Nightsister's possession". Obviously the circumstances are not quite the same as what is being discussed, but that is precedent for possessing bodies nonetheless. I mean Ezra straight up ghostbusts Sabine.
I think the reason it comes back is because it was an explicit plot point in the EU with Palpatine, as well as the fact that "dark wizards" possessing bodies to live forever is a well-worn trope, but obviously that doesn't count as precedent, only why fans might immediately go there mentally. Immortality is a recurring thing canonically, though, and it's suggested that it was a goal of Palpatine's master (and possibly Palpatine) and possibly achieved: "to cheat death is a power only one has achieved". The good guys achieve immortality through the force, but the bad guys can't, so it would make sense they would try to cling on it through other, more physical, means. Palpatine's speeches about cheating death are presented hand in hand (or just specifically relating to) creating life. So it isn't a huge leap on the theorization front.
However, because that is so typical, I have trouble seeing them actually go with that. Like, when you think dark wizards coming back, people these days will almost immediately think Voldemort. I don't think they want to just make space Voldemort, so I'm with you on it probably not being a likely theory. I mean could you imagine if Palpatine's spirit inhabited Sith artifacts and that was a pathway to return? Come on guys, though Star Wars isn't exactly original on that front.
