Picture books have been around from the very beginning. I'd say that the modern graphic novel is just the result of various gradual innovations, one of the important ones being that of embedding text - speech, thoughts, onomatopoeic description -
inside the pictures rather than alongside them, which improves the flow of the story, and which helps prevent the reader from becoming focused on either the pictures or text to the exclusion of the other.
I'd posit that anyone whose imagination conjures up mental pictures from a purely written book would be incapable of not doing so with a graphic novel. One's brain could have less to do, true... yet it doesn't choose the easy path. In my personal experience, the scenes of a good story grow and become more detailed in my imagination, regardless of whether they are depicted in prose or picture.
One thing I love about written language is that can be used in so many ways; the vast majority of reading isn't 'purely' reading for me, but associated with other activities - encyclopaedias are reading interspersed with skimming, possibly in order to inform another task such as writing, graphic novels are the appreciation of visual art in a narrative form supported by text, and then there's the entirely practical way in which being able to read "wet paint" helps me from getting myself into a sticky situation.