I would not say that Le Bon Marché is outside Paris core or this is a very restricted view of Paris core (kind like restricting New York City core to only Midtown).
It is within one of the most upmarket shopping area of Paris (Saint-Germain des Près) at less than a mile of the Louvre, at 10 minutes walk of Luxembourg garden.

North is down in this picture (bing aerial view of Paris are better in this way)
Le Bon Marché like the rest of the area is very upmarket.
Even if it is less touristy than the department stores on Boulevard Haussmann, it is not unknown or outside tourists sight.
It becomes more and more popular with asian tourists, especially Japanese.
Regarding the situation of department stores, France is more like the United States than Japan or Germany.
Paris is the only city in France with large thriving downtown department stores.
France is more a country of big box stores selling everything within it. The
Hypermarkets.
While in USA, department stores are still often anchors of shopping malls, in France it is the hypermarkets that act as anchors of shopping malls.