I don't know, I think some things are and should be sacred.
Do we want to see The Haunted Mansion replaced with an objectively better experience (a high bar, but lets assume they would clear it), or should the great new thing just be build in addition? What legitimate, compelling reason is there to replace something that classic, especially in WDW with its "blessing of size"?
What if Disney went to replace Cinderella Castle, even with something objectively better? Doesn't Cinderella Castle, for better or worse, hold the spirit of Walt Disney World in its image? Isn't it sacred?
Your mileage may vary on whether The Great Movie Ride exists in the pantheon with these examples, but I do think some things are sacred and that some things should be.
To me, and to others, The Great Movie Ride held the spirit of its park within it. I've always heard the reason it was closing was because it was more expensive to run than Disney liked - between the Animatronics, fire effects, and Live Cast Member involvement, the bills added up and they wanted something cheaper to run (which Runaway Railway is). Can you imagine a version of The Great Movie Ride that was reworked to remove the fire, minimize CM involvement, spruce everything up and replace 3 or 4 scenes in the name of keeping all of the good things about it? Sounds about as reasonable as offering Spaceship Earth its fourth major rework in under 40 years.
Runaway Railway, though fun, doesn't feel sacred to me. It's not a strong enough show to feel like "an instant classic" as some have suggested. I almost wonder if it'll last as long as The Great Movie Ride did. Artistically I think it shouldn't. It should see a serious update before lasting that long. Though that's maybe a silly thing to say about a ride that isn't even a month old, but its bones aren't nearly as strong as the ride it replaced.