I wonder what sort of square footage you Costco lovers have at home. As a downtown resident, I'd hate to fill up my small place with 24 packs of toilet paper, and I'm certainly not buying a second fridge. And if I decide to pay for another 200 square feet it won't be to save a dollar a pound.
The Costco concept also assumes you have a car. I've actually been asked whether I shouldn't buy a car, "at least so I can shop at Costco". Hysterical! Pay many thousands per year just so I can save a much smaller amount. I'll take my corner store, Pike Place Market, and occasional walks to Whole Foods or Metropolitan Market.
That said, I'm not against downtowns having big-box stores or even Costcos. While they may take some market share away from corner stores, traditional supermarkets, public markets, and locally-based larger stores, they can also help downtowns retain shoppers who would otherwise go elsewhere, including downtown workers, and people drawn from neighborhoods miles away.
Actually I'm looking foward to a big-box mixed-use development planned on the far southeast edge of Downtown Seattle, on the current Goodwill site at Dearborn & Rainier. It will apparently get a Target and Lowe's as well as a new Goodwill, other stores, community space, and 500 housing units. This would draw huge numbers of people from the underserved Rainier Valley, and the Target would sell things Downtown is weak on.
http://seattletimes.nwsource.com/html/localnews/2003463829_goodwill06m.html