setting up a character to serve as a villain for three films then killing them off partway through the second is very clearly different than setting up a character to server as a villain for three films then killing them off at the end of the third film
by the logic you seem to be using, 9 also didn't undo anything 8 did, which is patently absurd
Snoke wasn't set up to serve as villain for three films. He was set up to serve for two films, same as the emperor. You're just salty that your fantheory didn't come true.
if for some reason you believe it's impossible for a film to retcon things, why not just save us both some time by opening with that, rather than having this pointless little back and forth?
it's a really weird time to argue for death of the author when we're talking more or less specifically about directorial intent and we have the interviews from people involved
it's also kind of weird to argue for death of the author while also insisting the emperor was only established in 5, when he's mentioned multiple times in 4
It’s impossible for 8 to have undercut 7’s core themes, because 7 doesn’t have any.
8 didn't do any retcons. A retcon would be "actually Snoke isn't the main villain, he's just a clone created by Palpatine." 8 was perfectly happy to play nice with all the facts established in 7, and then have Ben kill Snoke as the first dramatic climax of the movie. 8 gets to have two dramatic climaxes because Rian Johnson is a brilliant filmmaker.