Walmart has competitors in every market I've seen. Small towns seem to be more likely to have chain dollar stores than a Walmart.
In my small city of 50k, Walmart doesn't exclusively cover any category other than maybe toys. There are other grocery stores, other automotive stores, other clothes stores, jewelry, drugs, blah blah. You go to Walmart because it's cheap, not because it's the only option.
So if the question is where do I go to get some random item where it won't break immediately and I won't overpay, Walmart seems like a reasonable choice.
Of course, Walmart is not alone in the category of wide selection, reasonably cheap, not terrible quality, Target comes to mind, and I think Kohls too, although I rarely go there so I might be wrong about that. Anyone know any others?
So I agree Walmart is not a monopoly, even if you are looking for apples-to-apples comparisons it is not unique.