Quote from Walt Disney himself at the Disneyland dedication ceremony:
"Before you enter this realm, I'd like to read this dedication -- which will be inscribed on a plaque:
'Frontierland: It is here that we experience the story of our country's past -- the color, romance, and drama of frontier America as it developed from wilderness trails to roads, riverboats and civilization; a tribute to the faith, courage, and ingenuity of our hearty pioneers who blazed the trails and made this progress possible.'"
Like every other land at Magic Kingdom, Frontierland is not and never has been about a specific decade or zip code. Frontierland is and always has been about
ideas -- "faith, courage, and ingenuity" -- taught to us by a broader time and place in history -- America beyond its original European settlements (which are already represented by Liberty Square). When you walk into Frontierland, you're quite literally heading west from polished colonial settlements into rougher untamed wilderness.
Did original attractions refer to specific years (1870s) and specific places West of the Mississippi? Absolutely. But that is just part of immersing us in specific stories. Jungle Cruise and Pirates of the Caribbean take us to wildly different time periods, and they do so purposefully to give us genuine-feeling experiences, but they exist together in a land that covers an even broader set of places and times that still feel extremely cohesive around core
ideas.
Frontierland can have individual experiences take us to specific times/places to
support the broader ideas Walt Disney described above without
restricting them. Riding off-road vehicles through "wilderness trails and roads" is absolutely consistent Frontierland and the pioneer feeling of venturing into uncharted/wild terrain, and gives us a different lens into its core ideas beyond the 1870s (which Splash Mountain / TBA expanded and re-expanded decades ago)
I think that Disney's push for IP lands has gotten people trained to think lands need to be about one ultra specific time, place, story, and set of characters. Disney's original lands were much deeper in concept, and I for one appreciate we're not getting "Cars Land" at Magic Kingdom, but rather, an expansion of Frontierland which are very different things.