Entrepreneur.com has an interesting list of the 10 Businesses Facing Extinction in 10 Years. No surprise that newspapers and pay phones are on there, but a few may be worrisome to those trying to revive urban business districts: Used bookstores, record stores, and (arguably) gay bars. These kinds of businesses, with low start-up costs and little need for advertising, once thrived in neighborhoods that weren’t quite affluent enough to attract national chains like The Gap or Urban Outfitters. I’m thinking in particular of Somerville’s Davis Square, considered a shining example of a vibrant neighborhood on a public-transit line with an economy based almost entirely on independent businesses. One of them is a used bookstore (McIntyre and Moore), and not too long ago there were two used record stores, all of them conducive to browsing. Can we nurture other Davis Squares without these kinds of establishments?