Except that proved you can, by doing exactly that with Clone Wars and Rebels, which retroactively massively improved how people felt and thought about the PT.
I'm afraid you're demonstrably wrong when you claim it "cannot" be done.
Whether they will succeed re: the ST? That's another question. But I suspect that in 10-15 years, if they keep at it, that plus nostalgia from people who are like, 15 now will make some major differences to how the ST is perceived. And those changed perceptions DO work their way back through culture. Gen X absolutely reviled the PT at the time, and for years thereafter. But even Gen X increasingly revised their opinions on it, maybe not to "It's cool!" to but away from "It's nothing but suck" as the generation who were like 10 when they saw it, and who grew up with Clone Wars etc. went on about how much they liked it.