I think as long as it remains profitable, they should keep it. The R&D costs are already well recouped so it's basically little more than a profit center. I understand Apple doesn't exactly need the money, but I think it would be a bad idea to kill off a licence to print money like the iPod.
Apple likes to keep a limited product lineup, so if a model isn't particularly necessary they can replace it with another product (such as an Apple TV variant), as well as consuming display space in stores.
