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.
https://www.barnesandnoble.com/readouts/the-wal-mart-effect-...
Step 1: Company has a great product. Walmart begins stocking it nationwide, perhaps promoting it to increase their marketshare. That product's revenues explode, they expand, everything seems great.
Step 2 to N: Then Walmart threatens to pull from shelves unless they can meet the cost of $substituteGood. The company cuts costs to make it work. Rinse and repeat.
Step N+1: The product is no longer great.
This is exactly what has happened with Rubbermaid and with countless other consumer goods.
https://www.amazon.com/United-States-Wal-Mart-John-Dicker/dp...