Yeah, she was a mostly forgotten, barely mentioned character and after being unusued for 20 years, everyone was crazy for her and hates to see her being "destroyed". I bet as much as you want that nearly all the people complaining would be mad if Hickman made an original character so relevant and say they should have used an existing one instead, someone like...Moira.
My two cents: I still see Moira in Moira X. What's been done to make her a mutant and retcon her storyline is done, so what we have left is the characterization.
I still see a driven, stubborn, brilliant, and unstoppable woman. Moira X to me is Moira MacTaggert driven to the brink of madness with what she's had to endure. At the core, she still wants to help mutantkind and that's really the essence of her character. (She was always the person, i.e. plot device, that the X-Men went to for help.) Only, she has become this sort of mad scientist version of herself.
I'll have to re-read Inferno #4 and I don't want to give any spoilers away, but I haven't completely dissected that last thing she revealed about her intentions. But I still think it fits.
I said I was trying to go for no spoilers lol!
It's my opinion that it still fits. One of Moira's lives was devoted to that and that's when Destiny burned her, IIRC. I think it was her second life and it was a misguided attempt (much like Wanda) to "solve" the problem. Now in life X, she's at her "wit's end", so to speak, and is going back to that insane thought process. Like I said, I haven't personally figured out what I think of what she said in Inferno #4. Gotta re-read!
No, that's not development.
Character experiences something. Character changes as a result of those experiences. THAT is development.
Taking decades of history and storytelling clearly establishing Moira as a baseline human and steadfast support of Xavier, and then simply FLIPPING A SWITCH BY EDITORIAL FIAT to "Oh, she's really a mutant, and has been all this time" is NOT development.
I said the core of the character is that she wants to help mutantkind. Her efforts to cure them were born out of that desire. And again, it's just like Wanda thought she was helping with NO MORE MUTANTS. You're just looking at it in black and white so that you can just say no to everything.
Moira being a mutant is not more of a retcon than Moira existing and being Xavier's right hand woman despite the fact neither Xavier nor the X-men met, mentioned or talked about her for 60+ issues. Or Xavier and Magneto being buddies and we only discover this after 20 years.