John Adams said: ""The Devil Wears Prada" was so good as a moviebecauseit was a movie.
Meryl Streep's and Anne Hathaway's entrances in the film - as only one "for instance" - are so exciting, and establishexactlywho these characters are because of the direction and editing techniques used that are uniquely cinematic.
The casting was perfection, also. The performances created by Streep, Hathaway and Stanley Tucchi in particular are so perfect that eventhey, themselvescouldn't recapture them for "...Prada 2". (IMO, they'd all "aged out" of the characters, and apparently their fictional characters hadn't benefited from any of the life experiences gained by living/working in the world for an additional 20 years.)
To try and recreate the same affection audiences have for the original movie, in a completely different medium, is a HUGE undertaking.
Whoever is cast in a staged version has to overcome the obstacle of making an audience forget enough about the original performances to accept what's currently on display as being at least equal to, or better. They also have to justify how singing and dancing their thoughts and dialog is an improvement over the cinematic version.
The same is true for the director of this musical, but they do not have the cinematic toolbox of techniques thatDavid Frankel used to make the movie a success. The director of this musical version also has to create something that's equal to, or better than the original.It's not an impossible task, but if it fails to meet the brief, it comes off as hubris, or insulting.
Sometimes it's better to just leave what's good untouched."
So you haven't seen it, right? Because you don't mention that, and almost everyone on this board who has seen it talks about how good Williams is in the role, and that the show--while nothing special or original--is fun. Your theory of why adapting a movie to the stage often doesn't work is pretty spot on, but you aren't saying that you've seen this and it bears out your thesis. Sometimes things work out.