I'm going to give a serious reply to this because I believe in sharing perspective and basically having an informative discussion. The main reason why people are upset is because the Ant-man movie for years has basically been Edgar Wright's baby. This wasn't a case of Marvel wanting to do an Ant-man movie and picking him, and then having a falling out.
What happened is that Edgar Wright felt he had a really good idea for an Ant-man film, and pitched it to Marvel. They liked it, and they allowed him to continue developing ideas to convince them further. He eventually shot a test clip to show off how he intended to shoot the action and effects for Ant-man's powers, and he seemed to have a very attractive idea for the story of the film. From the start the entire concept of an Ant-man movie has been attached to one creative force - Edgar Wright. Marvel waited for his other films to complete so he would have time to shoot this, pushing the movie into Phase 3. Everything was done to accommodate the possibility of seeing his unique vision of the character done on screen.
Now all those years of waiting and all the promises have gone down the drain. Whatever it will be replaced with will be something different. For anyone following the production of the movie from a creative perspective, it's extremely disappointing and sad.