The only successful shops I can think of, such as Amazon, Steam, etc., aren't successful due to DRM (at least, not from the piracy perspective). It's due to an easier shopping experience, with the DRM getting out of the way. I'm not sure you can even meaningfully call Amazon MP3's approach DRM: it's just watermarking.
Pirating content is trivial and easy. In order to compete, you have to make an as-nice experience. Buying a book on the Kindle store is _easier_ than torrenting the book (if only slightly) - DRM has nothing to do with this aspect. DRM doesn't even remotely stop the spread of content.
The only time I've heard DRM being remotely effective is really complicated systems on games, delaying a crack by a week or so, which apparently increases sales (at the cost of plenty of bad-will from paying customers) - although I'm not aware of any published studies that really investigate that and compare to the negative sides. This is the only time your analogy of medicine delaying death makes sense; one small use case, and only for specific programs, not general content.
But for content? DRM is a non-solution, really.