It’s kind of a meme; Costco’s profits are almost exactly the same as their total revenue from membership fees, which leads people to think that the warehouses run at zero margin and the fees are their only profit source. The fees certainly give them room to run the sales at extremely low margins (though large grocers like Kroger only have something like 3% margins), but it wouldn’t take a huge shift in purchasing patterns to change this coincidence. If all the people who don’t use their membership that much dropped them and those who use them were all large-scale buyers, they would have to increase their prices just to give themselves a bit of cushion.