I agree with you in part in so far as they weren’t top billing characters, but they should have limboed them if they didn’t plan on using them for the foreseeable future. Killing them off for shock value and to further other characters stories was an error. I didn’t absolutely love Dreamer like some fans, I did like her, but there was those who did absolutely love the character who outright saw her death as purely to advance other characters stories and on that point at least it was.
Her death wasn’t a deal breaker for me watching the show and even Lorna’s death for me would not have been something I would have stopped watching the show over. Not being true to their own characterizations and writing down to the audience for myself is more along the lines of what is for me was sort of a deal breaker. The writers should have taken greater care all around to not treat their characters and their characterizations like to use a Hellfire Club analogy... chess pieces.