I personally believe it's a dangerous thing to simplify the anger at this announcement to simply "fear of change". Yes, the Disney Parks fandom has a long history of pessimism towards new things, but I think that's partly a perception thing, as when you think about it, our fandom is huge and made up of a massive diversity of opinions and tastes. It isn't that the entire fandom is hateful towards everything, it's that each new announcement will likely annoy at least one loud person in the community. The rest of us may instead feel positively towards it. We're not all hypocritical forever-haters.
Here, though, the announcement is of the unofficial death of Frontierland, a core castle park land, or at least the main theme of the land. Let's be real here, Cars does NOT fit into Frontierland at all, unless you are to change the land's theme to be a more basic read of the land's title, which is dumb and reductive ('Frontierland' is now the land of literal frontiers and environments, no longer about a point in time with its people and values and history that affected the future). Heck, even insiders were saying before D23 that the IP picked to replace the ROA "didn't fit the land at all".
I personally don't grieve the loss of the Rivers and (especially) Tom Sawyer's Island as much as other people do, although I do think Frontierland will be hurt massively for not having the Liberty Bell travelling around and adding much-needed kinesis and placemaking to the environment. But I am furious about Cars. I know I can't stop it, and I even like the Cars films, but we all have to be honest with ourselves here and see that this announcement was the first bad omen, the first warning to the future where it will only get worse from here. You can put any IP anywhere. Theme is dead.
No land is safe from irrelevant drivel that only furthers the undermining of the implicit artistry of themed entertainment, that will guarantee that the art we love is ruined slowly, and we will once again be the laughing stock of the art world as just "the expensive holiday destination where vapid thrill rides bankrupt the mindless Disney adults who adore their favourite corporation". This is why I'm mad, why I think everyone is mad, even if they haven't all consciously gone fully down this train of thought yet. And therefore I think our anger is justified here.
Maybe I'm just a crazed fool screaming at clouds again. I have some self-awareness at how pretentious this all sounded lol, I could just be over-reacting here to a new theme park ride announcement. Time will tell!