Amidst all of the hype of the 30th anniversary of the release of the first (or is it fourth?) Star Wars movie, there was a documentary
on-line at the History channel and on TV. My son wanted to watch, and so we indulged. Guess who made an appearance? None other than our esteemed Peter Jackson.
He was interviewed and added his two cents about the Star Wars movies. What caught my eye was, in the same documentary, the 'story' of Star Wars was examined. It was noted that SW followed the classical hero story formula (not the exact words) where the hero is trained by a wise old mentor
who then must fade into the background so that the hero can come into his/her own. This fading can be accomplished via the mentor's death, so that the hero
must take his/her place without the crutch/aid of the old bearded one.
Peter Jackson surely knows of this story formula, and so may have seen the diminishment of Gandalf as essential to the story arc of Aragorn. Unlike in Tolkien's view (or at least my view of the same), Gandalf does not fade until Sauron falls. PJ's Gandalf peaks somewhere in Fangorn, and begins his slide there, which is about the time Lord Aragorn starts bossing Theoden around.
It all makes sense now, and so having Gandalf destaffed by the Witch-King shows demonstrably that that mentor's days have ended.
And speaking of formulas, mathematically speaking, if WK>Gandalf, and Eowyn>WK, and Aragorn>Eowyn, as noted
here, then Aragorn>WK and subsequently, Aragorn>Gandalf.