There has always been "creative interpretations" of where attractions go and nobody has complained in the past because it's just where they always have been. Frontierland has both The Country Bears and Splash Mountain which is based off of Song of the South. Explain to me how the south and country music attributes to late 1800s western frontier expansion? Just because you find bears out west doesn't mean they should be singing country music. It's not "western" music, it's country music and that's the south. Splash mountain is an excellent ride, but it has nothing to do with the frontier, nothing. They both look like they fit in because people aren't over thinking it. Haunted mansion in Liberty Square? What does it have to do with colonial America? Even the building it's modeled after was built in the 1870s, a hundred years after colonial times. Are the ghosts inside it former colonists from the lost Roanoke colony? Oh and why is Swiss Family Robinson in Adventureland, aren't they dreaded movie characters that everybody is completely against? Isn't that fantasy?
This is different though, This is tomorrowland, and nothing has ever been out of place there right? The gas powered race cars going 3 miles an hour are a perfect fit for "tomorrow." How about "If you had wings" what did that have to do with tomorrow? Didn't airplanes first fly in 1903, or nearly 70 years before WDW? You're going to tell me Buzz Lightyear is not more in tune with the theme of "tomorrow" than any other attraction that had ever been in it's space? Carousel of progress has nothing to do with "tomorrow" at all, never did. It has to do with people in the past thinking their present is fantastic and couldn’t get any better. Even the final scene is always meant to be "our" present, though now even that's 15 years outdated. Mission to the moon, something that happened 2 years before the park opened, and if you're not aware, things that happened before are the past. How about the original movies that were in there? Wasn’t it just scenes of America in the 1970s?
Monsters Inc is sci-fi, and sci-fi is the theme they were going for with the new Tomorrowland. Yes it's not a 100% perfect fit, but only Fantasyland has nothing you can say is out of place because ANYTHING can be fantasy. I'm much more concerned with having a new attraction which I will like, one that's fun then saying it's not "tomorrowish" enough. I love Splash Mountain, Haunted Mansion, Carousel of progress, and hell even The Country Bears (though Swiss family could burn down and I wouldn’t shed a tear.) I don't complain that they are not properly themed, they're fun, and fun is what I am concerned with. No average guest is going to complain it's out of place, they will complain however if what is put there completely sucks. Quality is what needs to be worried about first, then worry about how to theme it loosely enough to fit it in. Don't overthink the theme element, you didn't in years and lands past, just complain if the quality of the attraction is poor.