I feel that seeing the Sith fleet rise right after Luke restores the spark of hope in the galaxy would negate his heroics to soon after his sacrifice.
There never should’ve been a continuation of TLJ. Ever. The canon timeline, not just the main saga, should’ve just ended at the boy looking at the stars with hope. Period. End of story. Zilch.
Continuing the story and creating any new threats AFTER TLJ strips away any emotional impact from Luke’s sacrifice and inspiring the galaxy with hope, even if TFA and TLJ, themselves, remain untouched. Because what was the point of Luke’s sacrifice if TROS or DOTF or whatever continuation even exists? It was all for nothing since Palpatine came back anyways in TROS, or Kylo was gonna search for some planet-destroying Force ability in DOTF or the Resistance struggled against the FO in both Episode IX’s.
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Nah, I’m just kidding. Do you not understand that everything is temporary? You really think it only matters if it lasts forever? By your logic, TFA destroys ROTJ’s ending by destroying the Republic and Jedi and resurrecting the Empire. Logan destroys the previous movie’s ending by turning the world into an apocalypse and killing off all the mutants. Civil War destroys The Avengers from 2012 by having them break up. Nothing ever lasts forever. Everything is always temporary.
Besides, TLJ was always gonna have a sequel.