With The X-Files, we have another instance of a show trying to live on past one of its main characters and paying the price. The entire chemistry of the show rested on the balance of the relationship between Agent Mulder (David Duchovny), the true-believer, and Agent Scully (Gillian Anderson), the skeptic. Though lingering questions around key plot points and Scully's drifting away from her skepticism began to irk longtime fans as the show moved forward, die-hard X-philes hung on for the ride.
A show with a foundation so firmly built on a relationship between two characters can hardly expect to survive one of their departures. Sure, the Agents who joined the show after David Duchovny left a moldering (see what we did there?) hole in the series were game, but they couldn't hope to replace the seven-year relatoinship between belief and skepticism that comprised the show's fundamental fabric.