It's obviously a demotion for THE Hayden Panettiere to get kicked out and fired from voicing the female lead of the KH series, to then voicing some minor character. That's like firing Keanu Reeves from playing Neo in the Matrix, then telling him that they hired someone younger to pay Neo, but they'll bring Keanu on as Bank Robber #2 or something. Total insult. It's not the same as Haley playing both Sora and Vanitas. If Hayden played Kairi and Strelitzia, that would be different.
Strelitzia seems to be a major character in KH4 and initial reactions towards her have been positive. Also, let's be real, Kairi's screen time is very, very limited. I bet most casual players of the series would barely notice that her voice actress got replaced. While, yes, she is a major character, she hasn't been impactful enough in the story compared to Sora, Riku, and even Aqua. I bet people would be way more upset if Willa Holland were replaced because Aqua has had actual character arcs that people explicitly praised her for. By your logic, wouldn't it be a demotion for Hayden to voice Kairi to begin with? Considering how Kairi has been mostly a damsel in distress with no character development besides arguably the end of MoM? Promoting Hayden to potentially more interesting female characters in the series would also do her justice, which I believe is what Chaser and Sign have been saying.
If Square/Nomura were sly enough to pull off what you said in your first statement, then that's just sick! They trick Hayden into thinking they only want her for one line in 0.2, but in reality there was a package deal of KH3/ReMIND/MoM's starring roles hidden inside? There's no way Hayden would have turned down the starring role of 2 main console games and a DLC expansion that heavily featured Kairi. Again, she said that she didn't even get a call and was upset.
You are making very vast assumptions. I don't believe Nomura has been this evil guy who intentionally screwed Hayden over. You have to understand that Hayden has genuinely gone through difficult life issues that have made her unable to return to acting. She has had an inconsistent schedule, which is why Alyson replaced her to begin with. Also, I've looked at the tweet in question. All Hayden said, and you can prove me wrong, was "No one has asked me :/" and that was it. To say that she was heartbroken and upset about it is an assumption. We don't know her true feelings on the matter or how it went down, besides some implied disappointment.
I don't see the problem with having Stoner voice Kairi for one-liners and Hayden do the main games. It's like Tom Hanks and his brother for the role of Woody.
So you don't want us "demoting" Hayden, but the way you have talked about Alyson has been pretty demeaning. She isn't just Hayden's replacement, she is a legitimate voice actress who has gone a long way. Putting Alyson on a level of where she can only act throwaway lines and replace a higher celebrity is doing exactly what you have accused people of doing with Hayden.
Who are you to say that it's baseless to claim that Hayden wasn't super invested in the series? How do you know if she cared for the series or not? The fact that someone on Hayden's level came in to do one line for a handheld game shows me that she cares. Hayden could have easily said "let the other girl do the one line, call me for the real game", but Hayden showed up.
And you have no proof that she is invested and really cares besides your repeated point of voicing one line. We don't know for sure and you are unnecessarily offended on the behalf of a celebrity whose feelings on the matter we don't explicitly know about.