The PT and the OT were a father and son story, the origin of a villain, the birth of his son, and ultimately the heroes' journey of said son to bring back his father and both uniting to defeat Palpatine. While SW ANH could have been a stand alone film and the other two unnecessary, the development of the characters made it into a solid trilogy, with good character arcs.
Rey completely undermines the saga, not just because she's a Palpatine, but because the "final chapter" of the Skywalker saga was hijacked from the Skywalker family by a mystery character with practically zero growth. A merry sue, even if justified by the poorly constructed revelation of her ancestry. It would be like End Game giving Captain Marvel, Capt America's (Avengers Assemble) moment and Tony's final snap. A brand new character with barely any connection taking down the series main villain, while relegating the OG characters to spectators and cheerleaders. That's the "final" chapter of the Skywalker saga.