All long-running shows take occasional mis-steps. It's inevitable that mistakes will be made. Actually, a new show is more likely to make such errors, given the folks are inexperienced and have yet to truly get to know their audience. But even long-running shows with their proverbial 'finger on the pulse' can drop the ball.
Usually it's not the mistake that kills the show, but whether the show recovers from it. This is why I don't think Nathalie Kelley's departure (for whatever reasons) and the resulting decision to kill off Cristal/Celia did not "ruin" the show. The show handled the departure/death badly, and wasted opportunities that her death opened up for the other characters. The show would be much stronger if NK had remained; I think a lot of people here would agree with that, even if it's hypothetical. But when the writers were tasked with picking up the pieces after NK's departure, they made several poor choices, most of which involved a time-jump that allowed them to just toss "the pieces" aside and walk away. The writing seemed to be willing the viewer to forget Celia ever existed, or at least to say She's dead, gone; everybody's moved on and so should you. The set-siders may have had ulterior motives for adopting this tone, but even if the viewer is told months have passed, they haven't seen them pass for the characters. Seeing them have a party in the first episode back after we saw Celia shot and left for dead in a burning building was NOT how a show ought to deal with the death of a lead character. Having them barely mention her in ensuing episodes, and waiting until several episodes in before the characters learn (through confession, not investigation) that Claudia was the shooter made it feel like an afterthought. Celia's death should have been part of everyone's dialogue for the first few episodes of season two, and trying to find out who had started the fire and who shot Celia (were they the same person? Was there a possible assassin still lurking around? Would Alexis's role in Celia being trapped in the room be discovered and how would she cover that up?) should have had more screen time. It's impossible to predict, but I believe staying on that path rather than just hand-waving the events of the cliffhanger and going off in a different direction caused a disconnect in viewers' minds. The transition would have only needed three or four episodes to conclude Celia's story properly. Then and only then would we meet Cristal and learn how she tied to Celia, not this rushed introduction of "Real" Cristal that also gave the idea that Celia was a phony/fake who did not deserve to be thought about as a "real" person.