And really you could say that Discover killed it, because they lost focus on the core (or legacy?) business.

Still, department stores in the US are nothing like they were in their heyday.

Sears and Kmart both did so much diversification in the ~1980s! Discover, Allstate insurance, Dean Witter, Coldwell Banker for Sears (and Kmart had bought Borders, Builders Square, PACE, OfficeMax). If you trace the total return to a holder of Sears stock c. 1980, including the value they would have received in terms of stock in the spinoffs, the shareholders themselves may have actually ended up doing quite well in our timeline, even compared to one where Sears had reinvested all their profits into Sears, Roebuck & Co and maintained a more dominant retailing business as a result. It seems like no one really cared if the core business succeeded or failed, though.