I would have to agree with the previous posts.
When the mythology became overbearing and complicated, the series really took a hit. FOX milked the series for all it was worth, and it ended up hurting the series in the end. The series worked best when the mythology wasn't so forthright, but rather mysterious and broad. The "monster of the week" and other quirky episodes is what really gave the X-Files its cult status.
For Chris Carter and company, there is a need to attract a "new generation" of fans. The kids who grew up hearing about the X-Files, but never truly experienced it. For older fans, nostalgia will overtake any trepidation they may have about the series' conclusion. The series has been gone long enough that people will easily forget the muddled ending and will gladly welcome Mulder and Scully back. I think Chris Carter is doing the right thing, because the spirit of the X-Files was nowhere to be found in those last few seasons. But if Carter is able to scare us, make us laugh, and most importantly, make us think, the film will have been a success.