I wouldn't call it a gimmick when the business has been so successful for so many years. They target educated shoppers who want to buy quality at minimum markup and not think too hard about it. If I want a TV, I know Costco will have good ones at a good price. If I need socks, same. Their food is cheaper and better than Kroger. It's just a win-win for shoppers and Costco. The only tradeoff is selection and dealing with the crowds.

What you wrote sounds intelligent but belies an ignorance of the business model.

Parallel to that, I have to imagine Costco makes a lot of money off of impulse purchases, which are induced by uncertainty in the specific items they will have available, plus mutable store layouts.

Long ago in undergrad I took a retail marketing class and we did a field trip to Costco; the GM told us it was part of their policy to rearrange parts of the store occasionally so that you had to browse the entire place to check off your shopping list. This increases the likelihood that you stumble across new products. So it’s this combination of “best price/quality without decision fatigue” plus some impulse buying that works for them. The fact that they are figuring out the price/quality trade off for you up front probably also makes it easier to impulse buy with fewer regrets.

Yeah, this right here. I don't want to sift through thousands of options. Sometimes I just want a widget that is good quality and I don't want to get ripped off.