It is already impossible to prevent saving the image. Snapchat uesrs don't seem to care about that for the most part. What it does is create a whole new class of worthwhile photos. By sending via snapchat you are saying "this is just a throwaway image I wanted to send for a quick chuckle. No big deal." Whether the person takes the time/effort to save that throwaway image isn't really of any concern.

That's how I see it being used anyway. It's not being used with the expectation that there is literally no chance of the image being saved anywhere.