What is your source that price and quantity were unchanged for a decade?
It’s possible that the person you’re replying to has been shopping at Costco for that amount of time.
Not sure if that is one of them. But Costco does carry a few loss leaders where they don't raise the price, even though they are no longer profitable. The hotdog and soda deal for $1.50 and the roast chicken for $4.99 are two of them.