Closure?

'Closure' is such an American term. By definition, it implies an inability to cope with life.
Life is continuous and fluid. In life, closure is death.
Mulder finds closure only when he is able to accept that Samantha is dead.
Pillar, who has supported Mulder through the search, weakens and crumbles when having to carry the weight for himself. Pillar finds no closure because he is unable or unwilling to face the reality of the world in which he lives. In the real world, bad things happen to good and innocent alike.
We don't know what Pillar's son had to endure before death; we do know what Samantha suffered. Like Iphigenia, she is sacrificed by a father whose cause requires the life of a child. After suffering through six years of unspeakable tests, Samantha finally ran away. At fourteen, she died under mysterious circumstances. Or she was taken away in starlight. Either way, she is dead. The only closure we can ever hope for.
Nancy Berns, in 'Why People Hate "Closure"', claims:

Myth Slayers are angered by the notion that one can simply stop grieving. To them, closure implies an ending point in their grief; a place at which they can leave the pain and move on.

'Myth Slayers' are those who hate closure, according to Berns.
Maybe there are those who think as Berns' suggests, that an ending to their grief would somehow be a betrayal of the deceased, an abandoning of a loved one. An understandable feeling, to an extent. It's not why I hate closure.
Closure isn't a symptom of grief so much as a symptom of our broken culture. It is a sign of one's inability to cope with life alone. Berns' 'myth slayers' are more likely, if they were to examine their feelings honestly, to find that they must hold on to the deceased loved one because there is no one else in their life to fill the sudden emptiness. If they let go, they would find themselves adrift in a sea of loneliness.
Those who seek closure are actually looking for the same thing. Showing a degree of bravery, they have accepted their aloneness but would prefer it to be different. What they actually seek is openness.
We live lives closed off from one another in a way our ancestors would find bewildering. Now, a family unit is an enclosed order, separate from the world around them, protected by the monastery walls of their mortgaged home.
Closure isn't about coming to terms with the loss of a loved one. Closure is about the bereaved opening up enough to welcome others into their life. It isn't the loss of the loved one that demands closure, it is loss of a sense of belonging to a loving relationship. The only closure with any healing power is, in fact, openness.
'Closure' comes at an appropriate time in the whole span of the X Files. In 'Redux', Mulder has already declared that saving Scully was the thing he wanted most of all. In Season Six, we see them grow ever more close. Having always claimed to be a seeker of extreme possibilities, Mulder proves himself a man of his word, for what could be more extreme than Mulder opening himself to another such that her life becomes more important, more urgent, more real, than that to which he has dedicated his life?
Mulder is able to accept Samantha's death not because of some pishy notion of walk-ins, but because he has opened himself to another. He no longer needs define himself by one who is absent.
Mulder doesn't find closure; he finds openness.