If it were as wholly irrelevant as you claim, they wouldn't be going to extreme lengths to hide the Cars attractions from the rest of Frontierland. You can certainly make the argument that "frontier" is a much broader term than people are allowing for, and you can even suggest that it's okay to have different isolated subland pockets that explore different kinds of frontiers. However, Cars is not at all interchangeable with the other attractions in Frontierland in the way that you're describing because it's quite simply not visually coherent with the other elements of the area. Everything else is designed to be viewed together, but Cars is not. The Imagineers know this; that's why they're sequestering it away behind trees and walls.