What is the difference between the ST and a hypothetical sequel to LOTR, where the ring of power in the original story was a fake, such that that another small hero, a Wobbit from Wobbiton, can go on a similar quest to destroy the ring, and Sauron for real this time?
Well, SW isn’t based on a book or a comic or anything like that, so technically there’s no reason why you can’t change things or add to it retroactively. Also the emperor wasn’t really given much importance until the PT expanded his motives and abilities, so considering all that’s been added to the franchise in the decades since ROTJ him coming back really isn’t that strange.
Also I don’t get the whole “it undermines X-plot-point” argument. TROS doesn’s change any of the character stuff that happened in ROTJ, and going with the Middle-Earth analogy; Sauron was killed twice and just because they had to do it again later it didn’t undermine what Isuldur, Elrond, etc. accomplished a few millennia prior to LOTR.
Lack of originality aside, I don’t see how the ST breaks any in-universe logic.