It hurts Walmart as a business very little to stop selling Vendor X goods on their shelves when there are substitutes available. However, for Vendor X it's often the case that Vendor X cannot adjust their output as quickly as Walmart can take the product off shelves. That action, taking them off the shelves for even a short period of time while Walmart presses for renegotiating terms can result in layoffs, loss of revenue could result in lines of credit drying up. It's an existential threat to a company because they can't scale up and down at the speed Walmart demands.

If I recall correctly there's a record of this happening with Rubbermaid, and they responded to Walmart's demands for prices by cutting costs, cutting corners with quality, and ultimately sullied their brand name.