As Merriam-Webster explains, "grind down" is a phrasal verb meaning:
1: to make (something hard) smaller and smoother by gradually rubbing
off tiny bits
The old dog's teeth had been ground down by use.
2: to weaken or destroy (someone or something) gradually
Poverty ground her spirit down.
These people have been ground down by years of oppression.
So, the Great Plains have been reduced in size gradually over time, in the first sense. If the writer also intends to talk metaphorically about the Great Plains, as though they were a person or thing that could be weakened or destroyed, then the second meaning also would be applicable.
"Is it like the author thinks the Great Plains are prosperous but the area in question is quite stagnant？"
I think it is not so much about prosperity but about the original stature of the Great Plains. These are the "Great Plains", once so great and grand but now reduced gradually over time to wind, dirt and trash at the side of a highway, like an after-thought that nobody notices, where the main attraction are the common stores like Walgreens, which are nothing special.