I definitely understand why some WDW fans look at Disneyland and think "well why does it get to be and MK doesn't?", but I also think that line of thinking just kinda completely ignores the key differences between the two resorts.
Disneyland has retained much of its history for many reasons, the two most prominent being it was Walt's park and the primary visitor base are people who come often, likely as part of a longstanding family tradition of doing so. The appeal of Disneyland is its history. That's why people love it.
Magic Kingdom is a completely different story.
It started off as Disneyland clone (bigger in size, but arguably reduced in quality) and has gone on to be come the vacation center of the theme park world. It's audience is completely different, feels differently about everything. It's vactioners, people who don't come often, people who don't have longstanding sentimental attachment to really anything in the park, but especially not opening day stuff.
And as such, I really don't think that just because something was there when the park opened that that automatically makes it historically valuable or barred from being touched. Magic Kingdom absolutely has a history of it's own, but it's history is not residing in the attractions it shares with multiple other parks around the world. It resides in what you can only find there, in the things developed for it that offer it a unique identity.
Magic Kingdom just can't, and shouldn't, operate the way Disneyland does. I know that hardcore Disney fans, especially the ones who are WDW natives, think it should but it just can't. We all have to remember that we're a very small drop in the ocean of the amount of people who're visiting Magic Kingdom and what we want or like or dislike is nowhere close to what the average visitor may think or want. They can't just cater to us.
I don't think Cars belongs in Frontierland. I've been open about that since this was announced. That is my issue. Rivers of America going away? Not so much. It holds next to no historical significance to Magic Kingdom and holds very little importance to the park today beyond a nice view which is not, despite what some people want to say, guaranteed to go away just change.
Opening day attractions absolutely should not get special treatment at Magic Kingdom unless they're the original version or the only remaining version. If it doesn't fit one or both of those criteria, then it should be just as much an option for closure and redevelopment as anything else.
It may hurt to see some stuff you love go, and I totally empathize. I'm sure I'll have to say goodbye to stuff I really love in the park sooner rather than later. But the way I see it, it's more positive for the park in the long run. Magic Kingdom can continue to progress and find more of itself rather than just being a Disneyland derivative stacked to the gills with clones and experiences you can have other places.