Considering these movies are supposed
to all be related and similar in feel, allowing directors to do whatever they want is, honestly, a bad thing. Especially since nearly every single one of them would want to make it their
movie, with their
vision, and more likely than not wanting to "reimagine" it so that it's "hip and edgy."
Personally, I'd much rather have control in the writer's hands than anyone's (seeing as how a movie is, really, their
vision coming to life), though I can see why the producers need to keep things in line for something like the MCU.