Just because there might not be a Book 8 doesn't mean we can't continue chewing over Books 1-7. Right? Think about it. Just because Jimmy Buffet doesn't write any new songs doesn't mean a true blue Parrothead won't go to his concertts. Right again?
As I mentioned before, a writer is someone who doesn't know how to NOT write. So it may very well be that however badly the rest of us are hooked on Harry Potter, JKR may be even more hooked. If anyone can nag her into writing more books, it will be her own kids. I understand that at least one of them is a Potterhead. So we've go the home front covered.
So don't stress. Just sit back and enjoy the politically incorrect Magic Carpet Ride.