Chuck lasted so long in part because it was on NBC, a network with its fair share of problems. Between the writer's strike, several regime changes, and several shitty shows that fared worst in the ratings department, keeping Chuck seemed like a sensible option each year.
Still, in 2009 things looked dire until Subway got swept up in a fan campaign that resulted in the show being renewed with a catch: Subway would foot some of the bills, in exchange for the cast hocking footlongs during the episodes, ad nauseam. So we got more Chuck and with scenes like Big Mike's hilarious soliloquies on the beauty of a $5 footlong? No brainer!