Complex sentence: Adams stated that during the 1960s, the "Green Lantern" comic book was on the verge of cancellation, which gave Adams and O'Neill a great deal of creative freedom, since many thought the book would not survive anyway.
Split into simpler sentences: Adams stated that during the 1960s, the "Green Lantern" comic book was on the verge of cancellation. Therefore, Adams and O'Neill had great creative freedom, because many thought the book would not survive anyway.