Don't wholly buy the declining income argument. People still have access to credit and make use of it. Go into any Best Buy especially around the holidays and look at the long lines. Go into many homes including the so called "working poor" and you'll find plenty of up to date electronics, clothing, and so forth.
Retail sales in the USA actually is doing very well:
http://www.tradingeconomics.com/united-states/retail-sales-annual
What is dying/having a hard time are brick and mortar. The rise of Internet and online shopping is killing physical locations. Years ago people canned the idea shoppers would go online in great numbers. Feeling was consumers would want to see and feel merchandise as they had shopped mostly for ages since the invention of the department store. Turns out that was incorrect and becomes more so as each younger generation takes to online shopping.
It is important to remember how department stores came about in the first place. If you've seen Selfridges on PBS you know the story.
Marshall Fields, Macy's, Galeries Lafayette, Nordstrom, and the rest all came out of the same idea; give (mostly) women from the new and growing middle (and even lower) classes places to spend their husband's money. In the process these stores became destinations where ladies could "shop". That is gave them a legitimate and socially approved reason to get out of the house.
Late as the 1970's women like my Mother still got dressed up to go "in to town", down town, or whatever you want to call it when going shopping at a department store. She like other mothers could bring the kids who would find some sort of amusements provided (retailers weren't dumb....) while she shopped. Then maybe there would be luncheon or a snack, then you headed back home.
What department stores did was consolidate the various and many shops that sold various merchandise under one roof. No longer did you have to visit various tradesmen, or they have to come to you. Now everything could be found under one roof.
This whole model worked long as you had large numbers of women who remained at home or otherwise unemployed. That began to change in the 1970's and continues today. Aside from the small "Ladies Who Lunch" set most females are not only working full time, but have a family and other obligations. They just don't have hours free to loll around department stores "shopping". Here is where the beauty of online shopping comes in....