From the article (emphasis mine):
But Internet shopping alone does not account for the great retail apocalypse that we are witnessing. In fact, some retail experts estimate that the Internet has accounted for only about 20 percent of the decline that we are seeing. Most of the rest of it can be accounted for by the slow, steady death of the middle class U.S. consumer.
Dave Stockman’s book (highly recommended though spirit-crushing) says the number of “breadwinner” jobs peaked in 1998 and has trended downward since.