I believe XF still has a big following; even if "active" fans are fewer, those who remember it and value it as a good show are the large part of the iceberg. It has had a major influence in popular culture despite its waning ratings in its later life; what people remember is Mulder & Scully, and that's what they expect to see.
I think Carter & co are gambling big in this one, and they know it. Carder & Spotnitz have had time to think of every detail and take care of the finer points in the script. They're not aiming at a semi-success, like with FTF. And I think the media buzz will be good. Expectations are very high, but I trust that without the time restraints of a TV show, their creativity will have returned. We'll see how it develops, but that's my initial feeling!