Thank you Ron. Now I understand where my problem is.
I mistakenly thought that the story happened in two *separated* locations. SpendLess in location A and Colson's in location B. The correct answer said discount stores have been opening near Colson's (location B). In order to weaken the conclusion I must assume the same type stores will open in location A. That's why I questioned that "I don't see any reason to make further assumption that only *discount* stores will open there." *There* means location A, not location B.
Now I understand that both SpendLess and Colson's are located in the same place that is called *Goreville’s central shopping district*. So it's clear now that those locations (where the discount stores are expected to close) might keep vacant as there are already enough discount stores in the same shopping district.
I would be less confused if answer B were rewritten to:
B. Increasingly, the stores that have opened in the *Goreville’s* central shopping district since Colson’s opened have been discount stores.