Witch King vs Gandalf in the movie distorted a key point about the wizards: they were tasked to assist humans and elves against evil, but not to use their power to defeat the enemy on their own, or to rule over Middle Earth. Saruman tried do do both, and lost eternity for it. Peter Jackson understood that, but chose for theatrical reasons to sweep it aside, just as he did the enigma of the Old Forest and the (in Tolkien's own words) "most important chapter in the book" on the Scouring of the Shire (the author compared it to "the situation in [Britain] after the war").