I guess they felt that you after defeating Hitler, David Duke is an anti-climactic big bad.
Anyway, I find it interesting that while Dark Empire says that Sidious learned how to Essence Transfer from the Tedryn Holocron sometime after ROTS and the Plagueis novel says the Banites lost the knowledge of Essence Transfer as a result of Darth Gravid and that Plagueis wasn’t interested in Essence Transfer, the Rise of Skywalker novelization makes it sound like Plagueis taught Essence Transfer to Sidious.
It’s very likely that canon plagueis and canon bane are the same character
Bane having succeeded in transferring his essence from one body to the next for a thousand years before meeting palpatine