> having a hard time thinking of situations where a seller like Cheerios would be able to say, "no, everyone is going to offer our products at $2.50 for the next week."

Because MRP (minimum resale price) is often illegal (bit it's complicated https://www.ftc.gov/tips-advice/competition-guidance/guide-a...)

MAP (minimum advertised price) is more permitted, which is why Amazon often makes you put items in your cart to see the discounted price.