As much as this might sound weird, I feel like the brave sacrifice is an over-used trope in science fiction.
That's OK, SW thrives on old, overused tropes.

We know it's cliché, most of OT is at this point (albeit mostly due to everyone ripping off/parodying/referencing it). In fact, heroic sacrifice is one of the oldest tropes known to man. Doesn't change the fact that done right, it can be as effective as it was in ancient times.
Death of natural causes wouldn't do it, if only because she wasn't that old. Remember, Luke is her twin brother and still in good health. Humans in SW universe can probably live over well 100 if nothing happens to them. In TFA both Luke and Leia were about 50 at most. Yoda eventually died of old age, but he was ancient even at the time of Episode 1 (not to mention living on a planet with no technology to speak of).