If weight is the issue, then at some point they'll get a 15" notebook down to 3 pounds (zinc-air batteries, anyone?) and that'll obsolete the netbooks.
I think they might also have a niche market for the 10-year-old Johnnie and Janie crowd. Parents who have already spoiled their kids rotten with their own iPhones can give the brats a "tweener size" keyboard and screen.