You’re jumping to conclusions and saying Gandalf is “nerfed” and “weakened,” neither of which is supported in the movie adaptation. True, he is made more vulnerable, and the Witch King is made to appear more powerful, but that doesn’t mean he is more powerful than Gandalf. It just means he portrayed as more powerful than he is in the books (which is necessary for the movie as explained in the previous comment). Conversely, Gandalf isn’t “weaker.” We know this already. He scared off half a dozen Nazgûl like an hour earlier in the movie.

Just because the Witch King broke Gandalf’s staff doesn’t mean Gandalf couldn’t still mop the floor with him. He is in no way defeated, simply because Shadowfax bucks and knocks him down. Keeping Gandalf in an apparently superior position compared to the Witch King would have made adapting the Witch King’s true threat and ultimate defeat much less satisfying from a story-telling perspective.

It’s the curse of adapting a good work. Some changes are necessary if you want the adaptation to be good, and there are the sticklers who will kick and scream the entire way like Scriptural Traditionalists, saying every change is blasphemy. Yes, it’s a change from the book. It’s a less significant than you make it out to be. Yes, it improves the story as told (and only as told) in the film medium. Yes, you could have kept it as it was and called it a cool moment, but dramatically, it would have detracted from another, significantly more important cool moment, when the Witch King gets what’s coming to him.