They're not going to ditch a movie halfway through production like this. Ant-Man is the opening movie for Phase 3, not to mention if it's a success it would be proof they can push a concept like Ant-Man and have it work. Guardians of the Galaxy is one thing, but a lot of it's success will probably come from the fact that it's a space opera - it's not as overtly connected to the MCU as say, Captain America.
Ant-Man, however, is an Avenger. If Ant-Man fails, it may mean that future heroes (Black Panther, Ms Marvel, etc) won't get a chance because 'that Ant-Man film didn't work'. It's an important step for the MCU.