Note all the independent films that were nominated. Just about every studio now has or will have an independent division - from Fox Searchlight to Sony Classics. Producers are wonderful, compassionate individuals who would never cast aside the original creator. Unless, of course, the writer is not willing to collaborate in the manner specified by the producer. Seriously, writers have to also understand that movie making is a collaboration.
Most conflict occurs between the writer and the director.
The producer often acts as the decisive vote or sometimes as the referee. Often, these discussions are productive and this is why I would never cast aside a writer in the early stages. I encourage writers to fight for their themes and ideas and voice their point of view. A writer will be let go only if he or she agrees to changes and then writes just what they wanted in the first place. I would also love to witness Terry Gilliam giving script notes to James Cameron.