I'm afraid rapid obsolescence is the nature of the digital camera beast. Short of a fully interchangeable back, it's hard to see how an upgradable camera would be feasible. Even that option probably wouldn't help a lot, the back would cost much more than the camera component.
Film is very egalitarian. No matter what camera body you use, the lens and film, not the camera determine the resolution and quality of your film image. Digital is completely new ground for camera makers in that they can not only position a camera by its features, but also by image quality and size. Years of fast obsolescence and rapid depreciation are still ahead.