Movie studios have changed a number of movies to the point that they were unrecognizable and unwatchable. While there are no specific movies that come to mind, but Alfred HItchcock certainly had a right to be worried that the studios would interfere with his artistic vision. Considering how brilliant his movies were, he did the right thing.
Yes, I believe that movie studios interfere too much in the works of different directors. A movie is a great work of art and the message that is meant to be delivered is often skewed by the studios when they alter and cut out parts that the director was trying to get across to the audience. As a director, I would feel insulted if my work was altered and would demotivate me from creating my visions on film for the people.
Most movies require a lot of promotion and marketing to break even or make any money at all. Movie studios often do surveys and test screenings to refine a movie so that it will be appealing to a wide audience. There are still opportunities for independent movies to be made.
Movie studios do not interfere too much with the final product, because ultimately, they are the ones that finance the project. There is nothing wrong with being responsible for the artistic performance if you are paying the bill. The studio has every right to do what it thinks it has to in order to see a profit.