Excellent post muon. Let me ask you this then- do you think there ought to be a Plains States region (such as the one in my regions map) at all then? Or do you think the Mid-West goes right up to the Mountain West?
I've never thought it makes sense for the Mid-West to stretch all the way out to the Rockies. For one, the region becomes so big at that point, that it's unwieldy and unhelpful, because there is too much regional variation within it. For me, a good barometer is this- if the region is easily broken up into distinct and large sub-regions, then maybe it shouldn't be one single region, but multiple regions. Here's an example- could you put New England and the Mid-Atlantic into one "super-region" and call it the "Northeast?" Sure. But it makes a heck of a lot more sense to just split it into two regions.
Second, a Mid-West that large would seem to capture areas that do not have much in common with the "classic" Mid-West. Example- places like North Platte NE, or Wichita KS. What do these areas have in common with say... Cleveland? Basically nothing. To have one region encapsulate both seems just ridiculous to me. Thus why I made my plains region.
I grew up in Denver which is clearly a Midwestern city with a large share of its population from the classic Midwest states of Kansas, Nebraska, Iowa, Missouri, and some Illinois. It has nothing in common with Cleveland. If any areas east of the Mississippi are Midwestern, it would be areas along and south of the National Road.
Denver is not Midwest...it's Southwestern. I don't know a single person anywhere who would define Colorado as Midwest in any sense of the word.