We can stretch it, but we have to know when we've wandered into something entirely different. A genre is defined by the readers not the writers, but writers are free to choose what genre they write in. And they can often take readers along as they stretch the limits of any particular genre. But the readership may not accept the new dimensions, in which case the boundaries will stay where they are. All, as ever, in my opinion.
Debate is the life blood of art.