GMAT Critical Reasoning (CR) | There are far fewer independent bookstores

So this is how I attacked the question:

The question tells us that there is a reasoning error in the stimulus, and that it's an assumption that is NECESSARY for the conclusion to be drawn.

The stimulus:
Premise 1: Indie Bookstores have decreased because Chain BS increased
Premise 2: B/c of the shortage (ie. Indie BS decrease), the variety of readily available books has slowed.

Conclusion: "Chain bookstores' success has been to the detriment of book consumers".

So, before going